Add HTTP/2 pseudo-headers as known headers

[Tratcher]

https://github.com/aspnet/KestrelHttpServer/blob/526dfdb332b30fd6042d8ef8d1a2ba99cc654010/src/Kestrel.Core/Internal/Http2/Http2Stream.cs#L55-L66

It looks like these are being treated as unknown headers. Consider adding them to KnownHeaders. Note we'll have to special case them in the generator as the colon does not translate into a valid C# member name.

https://github.com/aspnet/KestrelHttpServer/blob/526dfdb332b30fd6042d8ef8d1a2ba99cc654010/tools/CodeGenerator/KnownHeaders.cs