[internal-branch.go1.24-vendor] http2: encode :protocol pseudo-header before regular headers

HTTP/2 requires that pseudo-headers (which start with : and are
used to pass information other than the regular request headers)
be encoded before all regular headers.

The x/net/http2 Transport's extended CONNECT support is enabled by
the user setting a ":protocol" header in the Request. This header
matches the pseudo-header that will be sent on the wire.

Ensure that the :protocol pseudo-header is sent before any regular
headers.

For golang/go#70728

Change-Id: I70de7ad524ab9457d6dfb61cb3fabe3d53c6b39b
Reviewed-on: https://go-review.googlesource.com/c/net/+/641476
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Antonio Ojea <aojea@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
(cherry picked from commit 445eead6062de643453d82a677cf4ca9c977568c)
Reviewed-on: https://go-review.googlesource.com/c/net/+/643256
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>