http2: accept requests without :authority pseudo-header?

As of 10.0.0, ATS http2 request parser rejects requests without the `:authority` pseudo-header. However, haproxy's http2 client will in many situations send requests that have the `Host` header but no `:authority`. There is an issue open about this on haproxy side: https://github.com/haproxy/haproxy/issues/2592. The underlying standardization issue seems to be different wording regarding conversion of a `Host` header into an `:authority` header in RFC 7540 vs. 9113, where haproxy hews to 7540 re: preserving http/1.1 semantics as far as possible when converting requests between protocols.

The haproxy issue is originally about Jetty, which apparently ended up adding support for `Host` as an alternative to `:authority`. What's the ATS view on this? Would it be reasonable to accept `Host` in lieu of `:authority` if the latter is missing?

Start by reviewing ATS's HTTP/2 request-parser behavior alongside RFC 7540, RFC 9113, and the linked HAProxy issue. Done means reaching and documenting an ATS decision on whether Host may substitute for a missing :authority pseudo-header, with the expected behavior and validation approach defined.

Written by the indexing model from the issue text.