Add handler for HTTP/2 PRI

[glaslos]

See for context: RFC

[Beeram12]

Hii @glaslos
I just wanted to ask regarding the flow to solve this issue

1. I have to write the spicy grammer(parser implementation) for HTTP/2
2. Implementing HTTP/2 PRI handler
3. Integrating with exisiting code
4. Update makefile
5. Write tests

Is this the correct flow to solve this issue.If you can add anything that would be very much helpful

[glaslos]

@Beeram12 This issue is an adjustment of existing code, not yet utilizing Spicy for protocol parsing.
Your entry point is here consider what you need to change here to handle the HTTP/2 PRI request. Then modify the HTTP handler to reply to the request correctly.