wgengine: send disco key via TSMP on first contact (#18215)

When we have not yet communicated with a peer, send a
TSMPDiscoAdvertisement to let the peer know of our disco key. This is in
most cases redundant, but will allow us to set up direct connections
when the client cannot access control.

Some parts taken from: #18073

Updates #12639

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

wgengine/magicsock/endpoint.go

@@ -80,6 +80,7 @@ type endpoint struct {
 	lastSendAny               mono.Time      // last time there were outgoing packets sent this peer from any trigger, internal or external to magicsock
 	lastFullPing              mono.Time      // last time we pinged all disco or wireguard only endpoints
 	lastUDPRelayPathDiscovery mono.Time      // last time we ran UDP relay path discovery
+	sentDiscoKeyAdvertisement bool           // wether we sent a TSMPDiscoAdvertisement or not to this endpoint
 	derpAddr                  netip.AddrPort // fallback/bootstrap path, if non-zero (non-zero for well-behaved clients)
 
 	bestAddr           addrQuality // best non-DERP path; zero if none; mutate via setBestAddrLocked()