Option to generate clusters at HTTPRoute/Rule/BackendRef level

*Description*:
By default, Envoy Gateway creates clusters at the HTTPRoute/Rule level. When filters are added to a BackendRef, clusters are instead created at the HTTPRoute/Rule/BackendRef level, which provides better visibility into which backend is receiving the routed traffic.

Instead of relying on filters, offering a direct option to create clusters at backendRef level would significantly improve backend traffic visibility.