Allow configurable restrictiveness for various header checks

We are using h2 in one of our rust services that are deployed on K8s and used in mesh with Istio. Since at the data-plane level Envoy is used, Envoy has an old behavior of passing authority header in the format of outbound|port||service-name to the service being called.

When a request from Envoy reaches our Rust service, it rejects the request due to a malformed authority header:
malformed headers: malformed authority (b"outbound|8081||limitador.rahul-test.svc.cluster.local"): invalid uri character. It makes sense since | is not part of the RFC for authority header but I feel this check should be configurable depending on feature flags or something similar.

Most libraries in other languages are pretty relaxed about these checks and thus work flawlessly so I feel it's one of the areas we can improve upon in h2.

Start by locating h2's authority-header validation and any related header checks, then inspect how configuration or feature flags are handled. Define the supported configurability and verify that the Envoy-style authority value can be accepted when opted in, while strict validation remains available and the behavior is covered by tests.