fix: wrong access token lifetime computation (FirebaseV1)

Overview

Previously, we used datetime.datetime.utcnow() which gives a datetime object without timezone information (offset-naive datetime object). This can lead to errors. The new recommended approach is datetime.datetime.now(datetime.timezone.utc), which provides a more accurate "aware" datetime object that includes UTC timezone.

This became important because the expiry time from the python-google-auth library is also an "offset-naive" datetime object. Using these two different types together caused incorrect lifetime calculations.

To fix this, we are now using offset-aware datetime objects for both expiry and now time values.