feat(flutterfire_ui): added getter 'size' to query_builder

[al4nnw]

Description

Adds ability to get the size of a query within a snapshot.

Checklist

• [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• [ ] My PR includes unit or integration tests for all changed/updated/fixed behaviors (See [Contributor Guide]).
• [x] All existing and new tests are passing.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] The analyzer (melos run analyze) does not report any problems on my PR.
• [x] I read and followed the [Flutter Style Guide].
• [x] I signed the [CLA].
• [x] I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• [ ] Yes, this is a breaking change.
• [x] No, this is not a breaking change.