Add trio example HTTP/2 echo server

[standy66]

Added example of HTTP/2 echo server that uses trio async library.

[Kriechi]

Hi! Thanks for sending this new example. Would you mind rebasing to the current master and giving it another look at and manual test. There have been some changes - just want to make sure your example still works the same as you intended. We recently deprecated a bunch of stuff and Python versions - so I hope this makes it easier!

[lovelydinosaur]

I'd suggest we can safely close this off as stale.