Add action to show a message when merging to the main branch

[rlazo]

No description provided.

[github-actions]

📝 PRs merging into main branch

Our main branch should always be in a releasable state. If you are working on a larger change, or if you don't want this change to see the light of the day just yet, consider using a feature branch first, and only merge into the main branch when the code complete and ready to be released.

Add the 'main-merge-ack' label to your PR to confirm merging into the main branch is intended.

[rlazo]

This action will be disabled after merged until it's useful. I haven't found a way of creating a disabled action.

[github-actions]

Unit Test Results

   395 files  +   344     395 suites  +344   19m 3s ⏱️ + 17m 29s
4 730 tests +3 756  4 708 ✔️ +3 734  22 💤 +22  0 ❌ ±0 
4 746 runs  +3 772  4 724 ✔️ +3 750  22 💤 +22  0 ❌ ±0 

Results for commit 57e366b. ± Comparison against base commit 4f41bde.

♻️ This comment has been updated with latest results.

[google-oss-bot]

Size Report ¹

Affected Products

No changes between base commit (81bf400) and merge commit (0f89861).

Test Logs

• Base (81bf400): https://github.com/firebase/firebase-android-sdk/actions/runs/3438853660
• Merge (0f89861): https://github.com/firebase/firebase-android-sdk/actions/runs/3438923247

1. https://storage.googleapis.com/firebase-sdk-metric-reports/XyH2jadPeA.html

[vkryachko]

Would it make sense to have this check require adding a tag to the pr and fail otherwise? this might make it easier to spot the action required to merge the pr

[rlazo]

Would it make sense to have this check require adding a tag to the pr and fail otherwise? this might make it easier to spot the action required to merge the pr

Done