feat: implement user schema management (#7416)

This PR adds the functionality to manage user schemas through the new user schema service.
It includes the possibility to create a basic JSON schema and also provides a way on defining permissions (read, write) for owner and self context with an annotation.

Further annotations for OIDC claims and SAML attribute mappings will follow.

A guide on how to create a schema and assign permissions has been started. It will be extended though out the process of implementing the schema and users based on those.

Note:
This feature is in an early stage and therefore not enabled by default. To test it out, please enable the UserSchema feature flag on your instance / system though the feature service.

cmd/start/config_test.go

@@ -75,6 +75,7 @@ DefaultInstance:
     LoginDefaultOrg: true
     LegacyIntrospection: true
     TriggerIntrospectionProjections: true
+    UserSchema: true
 Log:
   Level: info
 Actions:
@@ -86,6 +87,7 @@ Actions:
 				LoginDefaultOrg:                 gu.Ptr(true),
 				LegacyIntrospection:             gu.Ptr(true),
 				TriggerIntrospectionProjections: gu.Ptr(true),
+				UserSchema:                      gu.Ptr(true),
 			})
 		},
 	}, {