Adds ConfigEntry#withDefaultValue for easier chaining

This allows to chain the default value instead of using the setter or constructor.
Long keys/values can be put into individual lines which improves readability.

In the future, we might want to have a Builder class that contains all thise #with methods

Core/src/main/java/com/songoda/core/configuration/ConfigEntry.java

@@ -34,6 +34,9 @@ public interface ConfigEntry {
 
     void setDefaultValue(@Nullable Object defaultValue);
 
+    @Contract("_ -> this")
+    ConfigEntry withDefaultValue(@Nullable Object defaultValue);
+
     /**
      * @see #withComment(Supplier)
      */

Core/src/main/java/com/songoda/core/configuration/ReadOnlyConfigEntry.java

@@ -40,6 +40,12 @@ public class ReadOnlyConfigEntry implements ConfigEntry {
         throw new UnsupportedOperationException("Cannot set defaultValue on a read-only config entry");
     }
 
+    @Override
+    @Contract("_ -> fail")
+    public ConfigEntry withDefaultValue(@Nullable Object defaultValue) {
+        throw new UnsupportedOperationException("Cannot set defaultValue on a read-only config entry");
+    }
+
     @Override
     @Contract("_ -> fail")
     public ConfigEntry withComment(Supplier<String> comment) {

Core/src/main/java/com/songoda/core/configuration/yaml/YamlConfigEntry.java

@@ -50,6 +50,12 @@ public class YamlConfigEntry implements WriteableConfigEntry {
         this.defaultValue = defaultValue;
     }
 
+    @Override
+    public ConfigEntry withDefaultValue(@Nullable Object defaultValue) {
+        this.setDefaultValue(defaultValue);
+        return this;
+    }
+
     @Override
     public ConfigEntry withUpgradeStep(int version, @Nullable String keyInGivenVersion, @Nullable Function<@Nullable Object, @Nullable Object> valueConverter) {
         if (keyInGivenVersion == null && valueConverter == null) {