Support for HTTP/3?

[jalberto-ghub]

Any possibility of providing support for the new HTTP/3 spec?
There seem to be some native implementations already available.
Maybe something to have on the horizon.

[jerbob92]

We rely on other implementations to be able to make this possible. It looks like okhttp is not going to support it: square/okhttp#907

We could support it by replacing (the connection part of) okhttp by envoy, but that would mean adding an extra dependency.

Our iOS implementation uses NSURLSession and that seems to support http/3 already.