[Kestrel] Support for custom decoder in headers

[analogrelay]

Follow up from #17399

In 5.0 we want to provide a hook to use a custom decoder for headers. A System.Text.Encoding.Encoder is probably not quite sufficient since the codec may want to see the header name in order to make decisions. A strawman API would be something like this:

string Decode(string headerName, ReadOnlySpan<byte> headerBytes);

[stephentoub]

A strawman API would be something like this

Would it make sense to be more like:

Encoding? GetEncodingForHeader(string headerName);

?

I'm thinking that would enable:

• more flexibility in the future, given the various APIs on Encoding
• the ability to cache an Encoding to use for a given header, which could in turn enable caching opportunities for a given input sequence
• potential devirtualization opportunities

Just a thought. Not sure if this has progressed since the original strawman.