FR: HTTP/2 CONNECT method

[cgrushko]

HTTP/2 defines a CONNECT method, where a single HTTP/2 stream is used to proxy data to/from an arbitrary TCP server.

It is described in https://datatracker.ietf.org/doc/html/rfc9113#name-the-connect-method

A rudimentary implementation is straightforward, though I don't know how compliant it is:

3d68e47

I tested my code against an example server described in https://nodejs.org/api/http2.html#supporting-the-connect-method

Testing this code requires updating MockWebServer, I assume.

[yschimke]

@swankjesse thoughts? The impl seems quite clean.