http3: add http3 package for testing in std

This CL adds an initial http3 package that serves as a thin wrapper over
the exported functions in the internal/http3 package. Doing so allows us
to import and vendor the HTTP/3 implementation in x/net from std. This
will be used to run std tests against our HTTP/3 implementation.

To prevent users from unexpectedly using this package, only unexported
symbols have been added to the package, with linkname that only allows
them to be used by the net/http_test package in std.

For golang/go#70914

Change-Id: I6b420bb1eb07fd86174ee4c1ede43ef26a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/762041
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>
Auto-Submit: Nicholas Husin <husin@google.com>