Add SetPriorityMessageOnEstablishmentFunc, which registers a
PeerPriorityMessageFunc callback invoked when a peer's session keypair
is established or re-keyed for forward data transmission. The bytes it
returns are transmitted to the peer as a transport message.
The message is "priority" in two senses: it bypasses the staged packet
queue entirely, so it cannot be evicted by TUN-sourced packets, and it
is enqueued ahead of the keepalive/staged packets that follow keypair
establishment.
Updates tailscale/tailscale#20081
Signed-off-by: Jordan Whited <jordan@tailscale.com>
7a66fbee4a