Generalize h2cat sample into a reusable library

[Tratcher]

#14582 Copied lots of code from Kestrel tests to a sample to allow manually testing low level HTTP/2 behaviors in other servers.

Notes from the bug about generalizing this code:

This certainly meets its manual testing objectives. Here are some ideas for making it more broadly applicable in the future. E.g. I'd like to directly consume this as a library in Kestrel and HttpSys tests and remove the duplicate infrastructure.

Move the bulk of the code from the sample to a class library so other components can easily depend on it. Only Program.cs would stay in the sample.
Remove the internals-visible-to requirement, even if it requires copying more code out of Kestrel. Alternatively, those internals could be moved to a shared sources project that Kestrel directly consumes. Some of these internals overlap with the code we want to share with HttpClient anyways.