feat(firestore): adds COUNT aggregation query

[telpirion]

This PR implements the StructuredAggregationQuery feature for Firestore.

[snippet-bot]

Here is the summary of changes.

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• firestore/integration_test.go:1807, tag firestore_count_query

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[enocom]

return qs.Send(res)

With a return in each branch, you can also remove the return nil outside of the statement.

[telpirion]

Compiler doesn't like it when I remove the return statement, even when I add a return nil after the if in the first branch.