wgengine{,/magicsock}: add DERP hooks for filtering+sending packets

Add two small APIs to support out-of-tree projects to exchange custom
signaling messages over DERP without requiring disco protocol
extensions:

- OnDERPRecv callback on magicsock.Options / wgengine.Config: called for
  every non-disco DERP packet before the peer map lookup, allowing callers
  to intercept packets from unknown peers that would otherwise be dropped.

- SendDERPPacketTo method on magicsock.Conn: sends arbitrary bytes to a
  node key via a DERP region, creating the connection if needed. Thin
  wrapper around the existing internal sendAddr.

Also allow netstack.Start to accept a nil LocalBackend for use cases
that wire up TCP/UDP handlers directly without a full LocalBackend.

Updates tailscale/corp#24454

Change-Id: I99a523ef281625b8c0024a963f5f5bf5d8792c17
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/derp.go

@@ -725,6 +725,10 @@ func (c *Conn) processDERPReadResult(dm derpReadResult, b []byte) (n int, ep *en
 		return 0, nil
 	}
 
+	if c.onDERPRecv != nil && c.onDERPRecv(regionID, dm.src, b[:n]) {
+		return 0, nil
+	}
+
 	var ok bool
 	c.mu.Lock()
 	ep, ok = c.peerMap.endpointForNodeKey(dm.src)
@@ -745,6 +749,15 @@ func (c *Conn) processDERPReadResult(dm derpReadResult, b []byte) (n int, ep *en
 	return n, ep
 }
 
+// SendDERPPacketTo sends an arbitrary packet to the given node key via
+// the DERP relay for the given region. It creates the DERP connection
+// to the region if one doesn't already exist.
+func (c *Conn) SendDERPPacketTo(dstKey key.NodePublic, regionID int, pkt []byte) (sent bool, err error) {
+	return c.sendAddr(
+		netip.AddrPortFrom(tailcfg.DerpMagicIPAddr, uint16(regionID)),
+		dstKey, pkt, false, false)
+}
+
 // SetOnlyTCP443 set whether the magicsock connection is restricted
 // to only using TCP port 443 outbound. If true, no UDP is allowed,
 // no STUN checks are performend, etc.