MeshHTTPRoute with a MeshGateway targetRef is restricted to a system namespace

[tony-bondarenko]

Description

I've spent several hours trying to understand why I'm getting the following error when creating MeshHTTPRoute with MeshGateway as a targetRef:
The MeshHTTPRoute "test-route" is invalid: spec.targetRef.kind: value is not supported

Only after digging into a source code I found out that MeshGateway kind is supported only when MeshHTTPRoute is created in a kuma system namespace.
It makes sense to add a warning to documentation about this. I've looked through docs multiple times and haven't noticed anything about this limitation.

Also, having routes to gateway in another namespaces rather than system, sound as a valid use case to me: i have a separate builtin gateway deployed for a project in the project namespace. I'd like to let namespace users to register routes for their gateway rather than granting them access to kuma system namespace. May be it makes sense to consider this functionality.

[lukidzi]

Triage: Currently it's not possible to create a MeshHTTPRoute in a different namespace when topTargetRef is MeshGateway, since MeshGateway resource is Cluster scope xref #10014 . For sure we need to improve message and add a note to the docs.

TODO:

• improve kumactl message
• add information to the docs