cmd/tailscale/cli: allow remote target as service destination (#17607)

This commit enables user to set service backend to remote destinations, that can be a partial
URL or a full URL. The commit also prevents user to set remote destinations on linux system
when socket mark is not working. For user on any version of mac extension they can't serve a
service either. The socket mark usability is determined by a new local api.

Fixes tailscale/corp#24783

Signed-off-by: KevinLiang10 <37811973+KevinLiang10@users.noreply.github.com>

cmd/tailscale/cli/serve_legacy_test.go

@@ -860,6 +860,7 @@ type fakeLocalServeClient struct {
 	setCount             int                       // counts calls to SetServeConfig
 	queryFeatureResponse *mockQueryFeatureResponse // mock response to QueryFeature calls
 	prefs                *ipn.Prefs                // fake preferences, used to test GetPrefs and SetPrefs
+	SOMarkInUse          bool                      // fake SO mark in use status
 	statusWithoutPeers   *ipnstate.Status          // nil for fakeStatus
 }
 
@@ -937,6 +938,10 @@ func (lc *fakeLocalServeClient) IncrementCounter(ctx context.Context, name strin
 	return nil // unused in tests
 }
 
+func (lc *fakeLocalServeClient) CheckSOMarkInUse(ctx context.Context) (bool, error) {
+	return lc.SOMarkInUse, nil
+}
+
 // exactError returns an error checker that wants exactly the provided want error.
 // If optName is non-empty, it's used in the error message.
 func exactErr(want error, optName ...string) func(error) string {