derp, wgengine: let clients advertise an opaque app name to DERP servers

Add an AppName field to the DERP ClientInfo so DERP servers can
attribute connections to the application making them, primarily for
best effort stats purposes. The value is plumbed per engine instance
rather than via a process global, so a process hosting multiple stacks
can attribute each one's DERP connections separately:
wgengine.Config.DERPAppName flows through magicsock.Options and
derphttp.Client into the naclbox-sealed ClientInfo JSON. Old servers
ignore the unknown field.

There are no callers in the tree yet setting the name.

Updates tailscale/corp#24454

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: Ia7d3e9c2b6f8140e5a9d7c3b2e6f1a8d4c0b5e9f

derp/derpserver/derpserver.go

@@ -141,6 +141,12 @@ type Server struct {
 	debug       bool
 	localClient local.Client
 
+	// onClientInfoForTest, if non-nil, is called with each connecting
+	// client's key and ClientInfo. It is set (before the server accepts
+	// any connections) via forTest.SetOnClientInfo and is nil outside
+	// of tests.
+	onClientInfoForTest func(key.NodePublic, derp.ClientInfo)
+
 	// Counters:
 	packetsSent, bytesSent     expvar.Int
 	packetsRecv, bytesRecv     expvar.Int
@@ -1061,6 +1067,9 @@ func (s *Server) accept(ctx context.Context, nc derp.Conn, brw *bufio.ReadWriter
 	if s.debug {
 		c.debug = true
 	}
+	if f := s.onClientInfoForTest; f != nil {
+		f(clientKey, c.info)
+	}
 
 	s.registerClient(c)
 	defer s.unregisterClient(c)