gRFC G2: gRPC over HTTP/3

[James Newton-King]

I've created a gRFC for how gRPC would work over HTTP/3.

There is a lot of overlap with the content in https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md. For example, request and response headers are unchanged between HTTP/2 and HTTP/3. Rather than duplicate content, the proposal focuses on the differences between HTTP/2 and HTTP/3.

See:

https://github.com/grpc/proposal/pull/256

Comments welcome!