net/portmapper: fire an event when a port mapping is updated (#15371)

When an event bus is configured publish an event each time a new port mapping
is updated. Publication is unconditional and occurs prior to calling any
callback that is registered. For now, the callback is still fired in a separate
goroutine as before -- later, those callbacks should become subscriptions to
the published event.

For now, the event type is defined as a new type here in the package. We will
want to move it to a more central package when there are subscribers. The event
wrapper is effectively a subset of the data exported by the internal mapping
interface, but on a concrete struct so the bus plumbing can inspect it.

Updates #15160

Change-Id: I951f212429ac791223af8d75b6eb39a0d2a0053a
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

net/portmapper/select_test.go

@@ -163,9 +163,8 @@ func TestSelectBestService(t *testing.T) {
 				Desc:    rootDesc,
 				Control: tt.control,
 			})
-			c := newTestClient(t, igd)
+			c := newTestClient(t, igd, nil)
 			t.Logf("Listening on upnp=%v", c.testUPnPPort)
-			defer c.Close()
 
 			// Ensure that we're using the HTTP client that talks to our test IGD server
 			ctx := context.Background()