feat(firestore): onSnapshotsInSync API

[russellwheatley]

Description

This is still draft for the time being as execution order is wrong for Android. Please ignore until PR is open.

• new API for Firestore onSnapshotsInSync.
• replaces defunct PR after I messed up the git history.

Related issues

None that I know of.

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • Yes
• My change supports the following platforms;
  • Android
  • iOS
• My change includes tests;
  • e2e tests added or updated in packages/\*\*/e2e
  • jest tests added or updated in packages/\*\*/__tests__
• I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • Yes
  • No

Test Plan

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[russellwheatley]

There's a problem with Android. The onSnapshotsInSync event is correctly fired after the onSnapshot event.

But after the async operation of the snapshot serialisation, the onSnapshot event is emitted last:

This manifests in the onSnapshotsInSync firing before the onSnapshot event on the JS side:

We need to figure out a way to get the onSnapshotsInSync event to queue up behind the onSnapshot event.