Add aiohttp-session-firestore to third-party extensions

[dcgudeman]

Async Google Cloud Firestore storage backend, published on PyPI. https://pypi.org/project/aiohttp-session-firestore/

What do these changes do?

Adds aiohttp-session-firestore to the "Third party extensions" section in the README.

Are there changes in behavior for the user?

No. Documentation-only change.

Related issue number

Relates to #1178

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (6c77da5) to head (3bc7950).
⚠️ Report is 68 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1179   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           19        19           
  Lines         1481      1481           
  Branches        25        25           
=========================================
  Hits          1481      1481           

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