Add new firestore-nightly workflow

[wu-hui]

Example run: https://github.com/firebase/firebase-ios-sdk/actions/runs/7132339153/job/19424930017?pr=12180

Note the tests failed due to some composite indexes missing, and nightly seems to have large serverTimestamp drifts.

[tom-andersen]

This seems fine, with no customer facing consequence, so I will approve.

But how will we know this is working?

[google-oss-bot]

	1 Warning
⚠️	Did you forget to add a changelog entry? (Add #no-changelog to the PR description to silence this warning.)

Generated by 🚫 Danger

[google-oss-bot]

Coverage Report ¹

Affected Products

• FirebaseFirestore-iOS-FirebaseFirestoreInternal.framework

  Overall coverage changed from 88.10% (d724d30) to 88.08% (597056c) by -0.02%.

  Filename	Base (d724d30)	Merge (597056c)	Diff
  filesystem_posix.cc	81.82%	79.09%	-2.73%
  task.cc	94.78%	93.91%	-0.87%

Test Logs

• Base (d724d30): https://github.com/firebase/firebase-ios-sdk/actions/runs/7118908818
• Merge (597056c): https://github.com/firebase/firebase-ios-sdk/actions/runs/7133520036

1. https://storage.googleapis.com/firebase-sdk-metric-reports/7X0PfwzLvs.html

[wu-hui]

This seems fine, with no customer facing consequence, so I will approve.

But how will we know this is working?

The test outputs where it is connecting to, and the check is added as an optional CI workflow now.

[paulb777]

@wu-hui This job is both starting too late and consistently failing since added. Please address. https://github.com/firebase/firebase-ios-sdk/actions/workflows/firestore-nightly.yml

[wu-hui]

Nightly is unstable, i am going to make this workflow an on-demand one, instead of a scheduled or PR run.