HTTP/2: Improve incoming header performance

[github-actions]

This PR was automatically generated to sync shared code changes from aspnetcore. Fixesdotnet/aspnetcore#18943

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Issue Details

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This PR was automatically generated to sync shared code changes from aspnetcore. Fixes dotnet/aspnetcore#18943

Author:	github-actions[bot]
Assignees:	-
Labels:	area-System.Net.Http
Milestone:	-

[JamesNK]

Sync changes from dotnet/aspnetcore#38834

[JamesNK]

Changes:

1. Add StaticTableIndex to field entries. Used by Kestrel to set header values faster.
2. HPackDecoder uses the new OnDynamicIndexedHeader method to set a header using an entry from the dynamic table. If we know the header is set from the dynamic table then the name and value must previously have been validated. Because Kestrel uses that interface publically a DIM was added to keep it a non-breaking change.

Taking advantage of both of these changes are is opt-in. SocketsHttpHandler won't use it unless you choose to.

[JamesNK]

This PR has test failures from System.Net.Http.Functional.Tests, System.Net.Quic.Functional.Tests and System.Net.Sockets.Tests.

Is that because of changes here, or are they pre-existing known test failures?

[JamesNK]

Rebase fixed build.