Add support for configuring the plugin using environment variables.

Examples:
- Plugin.ServerName -> PLAN_PLUGIN_SERVERNAME
- Database.MySQL.Password -> PLAN_DATABASE_MYSQL_PASSWORD

Affects issues:
- Close #1353
- #1991

Plan/common/build.gradle

@@ -101,6 +101,15 @@ dependencies {
     testImplementation "org.testcontainers:nginx:$testContainersVersion"
 }
 
+test {
+    environment "PLAN_TEST_NODE_STRING", "String"
+    environment "PLAN_TEST_NODE_BOOLEAN", "true"
+    environment "PLAN_TEST_NODE_INTEGER", "5"
+    environment "PLAN_TEST_NODE_DOUBLE", "0.5"
+    environment "PLAN_TEST_NODE_LONG", "9223372036854775807"
+    environment "PLAN_TEST_NODE_STRINGLIST", "- Test\n- Another"
+}
+
 task updateVersion(type: Copy) {
     from('src/main/resources') {
         include 'plugin.yml'

Plan/common/src/main/java/com/djrapitops/plan/settings/config/ConfigNode.java

@@ -200,6 +200,14 @@ public class ConfigNode {
         return key;
     }
 
+    private String getEnvironmentVariableKey() {
+        String deepKey = parent != null ? parent.getKey(true) + "." + key : "";
+        if (deepKey.startsWith(".")) {
+            deepKey = deepKey.substring(1);
+        }
+        return "PLAN_" + StringUtils.replaceChars(StringUtils.upperCase(deepKey), '.', '_');
+    }
+
     public void sort() {
         Collections.sort(nodeOrder);
     }
@@ -255,32 +263,51 @@ public class ConfigNode {
         this.comment = comment;
     }
 
+    private String getEnvironmentVariable() {
+        String key = getEnvironmentVariableKey();
+        String variable = System.getenv(key);
+        Map<String, String> env = System.getenv();
+        return variable;
+    }
+
     public List<String> getStringList() {
+        String environmentVariable = getEnvironmentVariable();
+        if (environmentVariable != null) return new ConfigValueParser.StringListParser().compose(environmentVariable);
         return value == null ? Collections.emptyList()
                 : new ConfigValueParser.StringListParser().compose(value);
     }
 
     public Integer getInteger() {
+        String environmentVariable = getEnvironmentVariable();
+        if (environmentVariable != null) return new ConfigValueParser.IntegerParser().compose(environmentVariable);
         return value == null ? null
                 : new ConfigValueParser.IntegerParser().compose(value);
     }
 
     public Long getLong() {
+        String environmentVariable = getEnvironmentVariable();
+        if (environmentVariable != null) return new ConfigValueParser.LongParser().compose(environmentVariable);
         return value == null ? null
                 : new ConfigValueParser.LongParser().compose(value);
     }
 
     public String getString() {
+        String environmentVariable = getEnvironmentVariable();
+        if (environmentVariable != null) return new ConfigValueParser.StringParser().compose(environmentVariable);
         return value == null ? null
                 : new ConfigValueParser.StringParser().compose(value);
     }
 
     public Double getDouble() {
+        String environmentVariable = getEnvironmentVariable();
+        if (environmentVariable != null) return new ConfigValueParser.DoubleParser().compose(environmentVariable);
         return value == null ? null
                 : new ConfigValueParser.DoubleParser().compose(value);
     }
 
     public boolean getBoolean() {
+        String environmentVariable = getEnvironmentVariable();
+        if (environmentVariable != null) return new ConfigValueParser.BooleanParser().compose(environmentVariable);
         return new ConfigValueParser.BooleanParser().compose(value);
     }
 

Plan/common/src/main/java/com/djrapitops/plan/settings/config/ConfigValueParser.java

@@ -39,6 +39,8 @@ public interface ConfigValueParser<T> {
             return new BooleanParser();
         } else if (Long.class.isAssignableFrom(type)) {
             return new LongParser();
+        } else if (Double.class.isAssignableFrom(type)) {
+            return new DoubleParser();
         } else if (Integer.class.isAssignableFrom(type)) {
             return new IntegerParser();
         }

Plan/common/src/test/java/com/djrapitops/plan/settings/config/ConfigNodeTest.java

@@ -393,4 +393,52 @@ class ConfigNodeTest {
                     });
         }).collect(Collectors.toList());
     }
+
+    @Test
+    void environmentVariableString() {
+        ConfigNode node = new ConfigNode("Test.Node.String", new ConfigNode("", null, null), null);
+        String expected = "String";
+        String result = node.getString();
+        assertEquals(expected, result);
+    }
+
+    @Test
+    void environmentVariableBoolean() {
+        ConfigNode node = new ConfigNode("Test.Node.Boolean", new ConfigNode("", null, null), null);
+        Boolean expected = true;
+        Boolean result = node.getBoolean();
+        assertEquals(expected, result);
+    }
+
+    @Test
+    void environmentVariableInteger() {
+        ConfigNode node = new ConfigNode("Test.Node.Integer", new ConfigNode("", null, null), null);
+        Integer expected = 5;
+        Integer result = node.getInteger();
+        assertEquals(expected, result);
+    }
+
+    @Test
+    void environmentVariableDouble() {
+        ConfigNode node = new ConfigNode("Test.Node.Double", new ConfigNode("", null, null), null);
+        Double expected = 0.5;
+        Double result = node.getDouble();
+        assertEquals(expected, result);
+    }
+
+    @Test
+    void environmentVariableLong() {
+        ConfigNode node = new ConfigNode("Test.Node.Long", new ConfigNode("", null, null), null);
+        Long expected = Long.MAX_VALUE;
+        Long result = node.getLong();
+        assertEquals(expected, result);
+    }
+
+    @Test
+    void environmentVariableStringList() {
+        ConfigNode node = new ConfigNode("Test.Node.StringList", new ConfigNode("", null, null), null);
+        List<String> expected = List.of("Test", "Another");
+        List<String> result = node.getStringList();
+        assertEquals(expected, result);
+    }
 }