Support AppMesh Virtual Router, Routes, and Nodes

[armichaud]

The AppMeshBackend will be significantly more useful if the core functionality of the routing can be mocked. Namely, this would require implementing all CRUD methods for the virtual router, the virtual routes, and the virtual nodes:

• describe_virtual_router

• list_virtual_routers

• create_virtual_router

• update_virtual_router

• delete_virtual_router

• describe_route

• list_routes

• create_route

• update_route

• delete_route

• describe_virtual_node

• list_virtual_nodes

• create_virtual_node

• update_virtual_node

• delete_virtual_node

I'm planning on getting started on these soon – this issue is mainly just to advertise that I'm on it.