feat request: Add support for cross-namespace backends to ApisixRoute

### Description

For the apisixroute object in ingress-apisix, svc cannot support cross-namespace in the backend field. For example, the ns name is ait, the svc name is foo, and the svc address of backend is the configured foo.ait. In practice, 503 error occurs
In practical applications, whether due to pre-planning or other business reasons, some interrelated services are not placed in a namespace. However, the back-end of our site needs to access back-end interface addresses of different ns. I think configuration should be allowed

Start by tracing how ApisixRoute backend services are resolved and how Kubernetes namespaces are handled, then reproduce the described 503 with service foo in namespace ait. Determine the project entry points and tests for backend resolution. Done means an ApisixRoute can explicitly reach a service in another namespace without returning 503.

Written by the indexing model from the issue text.