Add [Query].[count()] to [FireStore] for [Android/IOS]

[baseemomar]

Add support for thecount()function in Query to retrieve the count of documents matching a query without fetching the entire data. This will return an AggregateQuery which can be executed using get(AggregateSource). This feature significantly reduces network usage and Firestore read costs, especially for large datasets.

[chris-hatton]

This is also the only functionality that's keeping my team from removing a direct KMP Cocoapods dependency on Firebase, which really slows down Kotlin builds. count() will be a welcome addition to the wrapping.

[chris-hatton]

Duplicate of #657