chore(api): initial definition of API for user schemas (user v3 and user schema v3 service) (#7372)

Adds the initial proto definition for a new user service (v3) based on user schema and it's corresponding user schema service (v3)

docs/buf.gen.yaml

@@ -5,4 +5,7 @@ managed:
 plugins:
   - plugin: buf.build/grpc-ecosystem/openapiv2
     out: .artifacts/openapi
-    opt: allow_delete_body
+    opt:
+      - allow_delete_body
+      - remove_internal_comments=true
+      - preserve_rpc_order=true