replace MeshGateway with a namespaced MeshBuiltinGateway

[lahabana]

Description

Maybe MeshBuiltinGateway is the right name.

Or maybe we have a general Gateway object to deal with provided gw too.

We'd need a way to restrict route attachment like Gateway API's allowedRoutes

[lobkovilya]

additional context from @michaelbeaumont

the one thing you can't do, either now or with namespaced MeshBuiltinGateway, is give permission for a strict subset of namespaces. like if I want to expose a gateway to route traffic to payments and orders I have to create a MeshGateway/kuma-system MeshBuiltinGateway, but either way, I still need kuma-system Routes. I can't give someone from payments/orders the option to only expose both payments/orders, so you always have to go through the mesh operator.

for the namespaced MeshBuiltinGateway to be truly more useful I think you'd need ReferenceGrant. so you could create a payments MeshBuiltinGateway, create payments routes and create a orders ReferenceGrant allowing refs from payments