Publish packages automatically after merge

[HakkyuKim]

Closes #378.

Summary

For pull requests events:

1. Checks for packages where version field in pubspec.yaml changed.
2. Run pub publish --dry-run to make CI fail on any errors or warnings. For example, missing a corresponding version entry in CHANGELOG.md.

For push events on master branch:

1. Hold off completing release workflow until all other workflows are finished.
2. Run pub publish --dry-run again just in case.
3. Publish the changed packages identified in the pushed PR.

Prerequisite

The content of the credential.json, which is created when running dart pub login, must be added to Github repository secrets with name "PUB_CREDENTIALS".

Limitation

New packages must be published manually because pub.dev current doesn't support publishing a new package directly to a verified publisher, See https://dart.dev/tools/pub/publishing#publishing.

TODO

• Create release workflow similar to https://github.com/flutter/plugins/blob/main/.github/workflows/release.yml.
• Fix step "Wait on all tests", it's not waiting for other workflows to be finished. -> The behavior was expected in a pull request event, I've confirmed it works on master branch. See doesn't work on pull requests lewagon/wait-on-check-action#25 for details.
• Test: pull_request event:
  • Changes without upgrading version. -> Expected: CI success.
  • Upgrading version without CHANGELOG entry. -> Expected: CI failure.
  • Positive case.
• Test: push event on master branch:
  • Check whether release is the last workflow to be finished.
  • Check whether the upgraded package is released.

[HakkyuKim]

We can put off setting PUB_CREDENTIALS to Github secrets and first see if the workflow runs correctly on the master branch, if it does, we can rerun the job after adding PUB_CREDENTIALS. I see google_maps_flutter_tizen isn't published for 0.1.2, shall I try with this?

1. Merge this PR.
2. Create a new PR that publishes google_maps_flutter_tizen-0.1.2.
3. Merge the PR and see if the only error is "PUB_CREDENTIALS not found".
4. Add PUB_CREDENTIALS and rerun the release workflow.

[HakkyuKim]

The flutter team allowed neutral conclusion due to Cirrus CI, it shouldn't be a problem on our side.

[swift-kim]

Otherwise overall looks good. Thank you!

[swift-kim]

@HakkyuKim You can test the push event with this PR: #392

[HakkyuKim]

@swift-kim
Okay, I'll merge #392 after this PR is merged.

[swift-kim]

https://github.com/flutter-tizen/plugins/runs/6586016243

The publication was successful after updating PUB_CREDENTIALS.