Add return type hint for get_watcher function

Co-authored-by: Lena Garber <114949949+lgarber-akamai@users.noreply.github.com>

linode_metadata/metadata_client.py

@@ -233,7 +233,7 @@ def _user_agent(self):
     def get_watcher(
         self,
         default_poll_interval: Optional[Union[timedelta, float, int]] = None,
-    ):
+    ) -> Union[AsyncMetadataWatcher, MetadataWatcher]:
         """
         Get a watcher instance with this metadata client.