fix: run credential refresh in thread (#508)

The async connector must sometimes refresh its credentials, but prior to this
commit the refresh operation would block the main thread.

Now that this library supports at minimum Python 3.10, we may use the
asyncio.to_thread method to run the refresh on a separate thread, awaiting its
result before continuing with the connection info refresh operation.

Fixes #245

Author: enocom

google/cloud/alloydbconnector/client.py

@@ -239,9 +239,10 @@ async def get_connection_info(
         """
         priv_key, pub_key = await keys
 
-        # before making AlloyDB API calls, refresh creds if required
+        # Before making AlloyDB API calls, refresh creds if required
+        # Run refresh in a separate thread to avoid blocking the main thread.
         if not self._credentials.token_state == TokenState.FRESH:
-            self._credentials.refresh(requests.Request())
+            await asyncio.to_thread(self._credentials.refresh, requests.Request())
 
         # fetch metadata
         metadata_task = asyncio.create_task(