allow to modify HTTP/2 header

[honfika]

Doneness:

• Build is okay - I made sure that this change is building successfully.
• No Bugs - I made sure that this change is working properly as expected. It doesn't have any bugs that you are aware of.
• Branching - If this is not a hotfix, I am making this request against the master branch

known issues:

• some http/2 validation is not enforced
• generated header is always 1 frame (can be bigger than a valid frame size), however I never saw any splitted header, yet
• copyHttp2FrameAsync method looks ugly, especially the encoding part, long