Faster generated headers enumeration for HTTP/2 and HTTP/3

[JamesNK]

@benaadams has a PR that improves writing response headers for HTTP/1.1:
#32337

The same concept can be applied to response headers generated Enumerator types. These are used when writing HTTP/2 and HTTP/3.

Writing response headers in HTTP/2 is a hotspot. Multiplexing and HPack means that only one response on a connection can write headers at a time while other responses wait on a lock. Reducing the time in the lock is huge.

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.