[AppMeshBackend] Support virtual gateway + virtual gateway route methods

[armichaud]

Enable mocking of the APIs governing ingress into an AppMesh via virtual gateways.

Although I do think that its best to wait on this until #8001 is fulfilled (it will probably borrow a lot from the work to enable virtual nodes and their routes), I've filed this as a separate issue since I suspect it's lower priority for people who are trying to test their AppMeshes.