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.

docs/sidebars.js

@@ -165,6 +165,7 @@ module.exports = {
           items: [
             "guides/manage/user/reg-create-user",
             "guides/manage/customize/user-metadata",
+            "guides/manage/customize/user-schema",
           ],
         },
       ],