Create Conformance Test Suite for Internal Encryption

/area API
/area networking
/area test-and-release

Describe the feature

In support of https://github.com/knative/serving/issues/11906
notes/discussion

We should cover the following scenarios:

• Basic test to call local ingress-gateway with TLS and check the returned certificate
• Basic test to call activator (without ingress-gateway) with TLS and check the returned certificates
• Basic test to call QP (without ingress-gateway and activator) with TLS and check the returned certificates
• Test with domain-mapping, check if the call to the ingress-gateway and local ingress gateway is done with TLS, check the returned certificates
• Test with http2, check the returned certificates

Depending on the implementation decisions, we should also add:

• Test to check if the ingress implementation is checking the upstream SAN with multiple values (activator SAN + QP SAN)
• Define and check what SNI information the ingress-gateway sends to upstream backends.

Some or all of these tests may need to also existing the Networking repo, since we will want to make sure ingress-controllers call and validate upstream connections correctly (so instead of activator/qp we can use a custom pod to check).

No files or test entry points are named. Start with knative/serving#11906 and the linked discussion notes, then determine whether coverage belongs in this repository or the Networking repository. Done means the agreed TLS, certificate, domain-mapping, HTTP/2, SAN, and SNI scenarios are covered.