Rework delegated gateway

[lahabana]

Description

Delegated gateway is a concept that we make up that is rather odd.

A Delegated gateway is a dataplane that probably doesn't have an inbound service in the mesh.
The is no actual need for having delegated gw a thing of its own. It just a regular dataplane with unmanaged ports.

This observation was already made as part of: #3339 (comment)

This would make things simpler it could be as simple as an annotation kuma.io/unmanaged-ports: 8080 and would solve problems where you want admin to be part of the mesh but not the regular listener.

This would remove the need for: kumahq/kuma-gui#3297 altogether as Delegated gw would no longer exist

[github-actions]

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