common/remotedatasource: add a Stop() method

This is cleaner this way. We can't use it for the static provider as we
cannot stop a provider.

common/remotedatasource/root_test.go

@@ -140,6 +140,7 @@ func TestSource(t *testing.T) {
 	handler.fetcher, _ = New[remoteData](r, handler.UpdateData, "test", config)
 
 	handler.fetcher.Start()
+	defer handler.fetcher.Stop()
 
 	// When not ready
 	handler.dataLock.RLock()