http2: add initial support for PRIORITY_UPDATE frame defined in RFC 9218

This change adds initial support for the PRIORITY_UPDATE frame
introduced in RFC 9218.

Clients can now use a new exported function to write PRIORITY_UPDATE
frames easily. However, sending PRIORITY_UPDATE frames to the server
does not currently cause any behavior changes: we only use
PRIORITY_UPDATE frames to adjust stream priority when the RFC 9218 write
scheduler is being used for a particular connection. However, this
scheduler is not currently usable yet from any configuration surfaces
exposed to the user.

For golang/go#75500

Change-Id: Ie2c821cb0d2faa6e942e209e11638f190fc98e2b
Reviewed-on: https://go-review.googlesource.com/c/net/+/705917
Reviewed-by: Nicholas Husin <husin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>