internal/http3: add TransportOpts argument to RegisterTransport

RegisterTransport now takes in TransportOpts, which allows
RegisterTransport user to specify how an HTTP/3 transport should open a
QUIC endpoint, and what QUIC configuration to use.

Practically, this allows us to access the QUIC endpoint that an HTTP/3
transport is using. This allows us to synchronously close QUIC
connections while exercising our tests in std, to prevent goroutine
leaks after running tests.

For golang/go#78737
For golang/go#70914

Change-Id: I34f9756040044624ca9ea21654b249526a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/768000
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>