[Conformance] BackendTLSPolicy as union feature

[rikatz]

What would you like to be added:

Now that the behavior of BackendTLSPolicy as a union feature is explicit on GEPs, API, etc we need conformance tests for these cases, where implementations must confirm they work with this combined feature

Why this is needed:

We need to confirm that implementations that claim combined support (eg.: BackendTLSPolicy with GRPCRoute) can actually achieve this

Proposed tests:

• An implementation that claims support for GRPCRoute AND BackendTLSPolicy MUST support the usage of BackendTLSPolicy attached to a service that is consumed as a backend by a GRPCRoute (similar to HTTPRoute tests)
• An implementation that claims support for TLSRouteTerminate AND BackendTLSPolicy MUST support the usage of BackendTLSPolicy attached to a service that is consumed as a backend, to re-encrypt the traffic
• An implementation that claims support for HTTPRoute, HTTPRouteRequestMirror AND BackendTLSPolicy MUST support the usage of BackendTLSPolicy attached to a service that is consumed as a backend of this filter

The tests for TCPRoute with termination and for External Auth should be covered by the GEPs of these new features (cc @youngnick @zac-nixon )

[davidesalerno]

I'm available and I would like to work on this test

[rikatz]

/assign @davidesalerno