Using a default HTTP timeout in db module

[hiranya911]

The google-auth package now sets a default timeout of 120 seconds for all outgoing requests (see googleapis/google-auth-library-python#435). This is causing a couple of test failures in our codebase: https://github.com/firebase/firebase-admin-python/actions/runs/32412728

But more importantly this change leads to unpredictable default behavior in applications that use firebase_admin. Specifically, developers who are on older versions of google-auth won't get any timeout (timeout=None), while developers on new versions will get a 2 minutes timeout (timeout=120).

This PR addresses the above discrepancy by setting a default timeout in our own code. This has the following advantages:

1. Developers using firebase_admin will see consistent behavior regardless of their google-auth version.
2. We get more control over the default timeout, and are no longer affected by the changes in google-auth.

Note that httpTimeout option is currently only respected by a subset of our modules. Other modules (e.g. auth), will get their default timeout from google-auth for now. I'm using the same default value of 120 seconds to mitigate this, but we should properly address this for all modules in a subsequent PR.

[hiranya911]

Closing in favor of #397