[FR] Count documents with aggregation queries

[BladeRunner713]

Is your feature request related to a problem? Please describe.
It seems like the new count() method mentioned on https://firebase.google.com/docs/firestore/query-data/aggregation-queries has not been implemented in Python SDK.

Describe the solution you'd like
Add the new method so that we can use the count method to count the number of documents in Firestore.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context, code samples or screenshots about the feature request here.

[google-oss-bot]

I couldn't figure out how to label this issue, so I've labeled it for a human to triage. Hang tight.

[lahirumaramba]

This is now fixed in #671
We will also update the public docs with code samples. Thanks!