conn,device: enable cryptorouting via PeerAwareEndpoint

Introduce an optional extension point for Endpoint that enables a path
for WireGuard to inform an integration about the peer public key that is
associated with an Endpoint.

The API is expected to return either the same or a new Endpoint in
response to this function. A future version of this patch could
potentially remove the returned Endpoint, but would require larger
integrator changes downstream.

This adds a small per-packet cost that could later be removed with a
larger refactor of the wireguard-go interface and Tailscale magicsock
code, as well as introducing a generic bound for Endpoint in a device &
bind instance.

Updates tailscale/corp#20732

device/noise-protocol.go

@@ -124,7 +124,7 @@ type Handshake struct {
 	localEphemeral            NoisePrivateKey          // ephemeral secret key
 	localIndex                uint32                   // used to clear hash-table
 	remoteIndex               uint32                   // index for sending
-	remoteStatic              NoisePublicKey           // long term key
+	remoteStatic              NoisePublicKey           // long term key, never changes, can be accessed without mutex
 	remoteEphemeral           NoisePublicKey           // ephemeral public key
 	precomputedStaticStatic   [NoisePublicKeySize]byte // precomputed shared secret
 	lastTimestamp             tai64n.Timestamp