ci(conflicts): create conflicts.yml workflow

[davorpa]

What does this PR do?

Improve repo

For resources

Description

The conflicts tag is commonly used to mark those PR that have merge conflicts / dirty status with the base branch. But this was a manually process.

Well, as part of enhacements requested in #7024 this workflow/pipeline automates that process using a GitHub Action, eps1lon/actions-label-merge-conflict, with which we...

• automanages the conflicts label
• post a message when conflicts are arisen.
  
• A summary report is composed on the fly with the mergeability results.
  

Workflow could be raised:

• manually (all PRs are checked. Useful as start up / for stucks)
• on push (all PR with linked base commit are checked)
• on pull request synchronize (current PR is checked)

Tests:

• mergeability 1 davorpa/free-programming-books#32
• mergeability 2 davorpa/free-programming-books#33
• workflow runs at: https://github.com/davorpa/free-programming-books/actions/workflows/detect-conflicting-prs.yml

Checklist:

• Read our contributing guidelines.
• Search for duplicates.
• Used an informative name for this pull request.
• Review message wording/spelling

Follow-up

• Check the status of GitHub Actions and resolve any reported warnings!

[eshellman]

I'm nervous about adding this so soon before October, even though it would be particularly useful before then.

[davorpa]

I'm nervous about adding this so soon before October, even though it would be particularly useful before then.

If it serve... is used by Jenkins (https://github.com/jenkinsci/jenkins/blob/master/.github/workflows/label-conflicting-pr.yml). I only add the GHA summary to report what happens in each run.

Of course, workflows can be disabled without revert code.

You decide.

@eshellman Could you review the comment text? Remove parameter is also possible

[LuigiImVector]

LGTM

[eshellman]

Commenting looks good. Jenkins doesn't get the volume of PRs that we do during October.