internal/http3: populate some http.Request fields

Currently, we are passing a very barebone http.Request to the server
handler: we only initialize an empty http.Request and put whatever info
we can get while decoding QPACK headers.

Unfortunately, this causes the Server to panic when parsing requests
whose headers are meant to be written to http.Request.URL, as
http.Request.URL was never initialized.

Therefore, make sure that http.Request.URL is initialized. Also,
populate other http.Request fields that we can easily figure out as of
now.

For golang/go#70914

Change-Id: Ie6552d6678b430fe4b51069616c0e366791c4e34
Reviewed-on: https://go-review.googlesource.com/c/net/+/738880
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>