support OCI-Subject header (#18885)

fixes #18865
the response header OCI-Subject to indicate to the client that the registry processed the request's subject.

Signed-off-by: wang yan <wangyan@vmware.com>
Co-authored-by: System Administrator <root@wangyanCQFQY.vmware.com>

src/server/middleware/subject/subject.go

@@ -129,6 +129,7 @@ func Middleware() func(http.Handler) http.Handler {
 					return err
 				}
 			}
+			w.Header().Set("OCI-Subject", subjectArt.Digest)
 		}
 
 		return nil

src/server/middleware/subject/subject_test.go

@@ -159,6 +159,7 @@ func (suite *MiddlewareTestSuite) TestSubject() {
 		suite.Equal(name, accs[0].GetData().SubArtifactRepo)
 		suite.True(accs[0].IsHard())
 		suite.Equal(accessorymodel.TypeSubject, accs[0].GetData().Type)
+		suite.Equal(subArtDigest, res.Header().Values("OCI-Subject")[0])
 	})
 }