ipn/ipnlocal,wgengine: implement wg-go SetPriorityMessageOnEstablishmentFunc (#20606)

Implements a way to send TSMPDiscoAdverts based on a trigger from
wireguard-go when a rekey happens. This lets us distribute disco keys
consistently, but also sets us up for a minimal message that can be
distributed to other clients.

A benchmark is implemented to make it easier to keep the call cheap and
to avoid locking up anything in wireguard-go.

Updates #20081

Signed-off-by: Claus Lensbøl <claus@tailscale.com>