tailcfg: add ServiceName

Rather than using a string everywhere and needing to clarify that the
string should have the svc: prefix, create a separate type for Service
names.

Updates tailscale/corp#24607

Change-Id: I720e022f61a7221644bb60955b72cacf42f59960
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>

cmd/k8s-operator/ingress-for-pg_test.go

@@ -137,7 +137,7 @@ func TestIngressPGReconciler(t *testing.T) {
 		t.Fatalf("unmarshaling serve config: %v", err)
 	}
 
-	if cfg.Services["my-svc"] == nil {
+	if cfg.Services["svc:my-svc"] == nil {
 		t.Error("expected serve config to contain VIPService configuration")
 	}