Create shared backport workflow

[pavelhoral]

We are planning on supporting the future Jakarta based implementation that will live in the default main branch as well as the current AM that will live in sustaining/15.x branch. This means that a lot of fixes and even features will need to be backported. To make our lives easier I would like to have the following:

• shared GitHub workflow (or Action or Bot) that can create new PRs based on PR label
• I would like the main part of the workflow to be implemented as a shared workflow in .github (in the same way as SonarQube workflows were made)
• the workflow will be triggered by:
  • closing PR, then checking for the presence of backport label
  • adding backport label to already closed PR
• the workflow will do the following
  • clone the merge base branch
  • create new local branch with the name backport/pr-{PR number}
  • somehow determine (not sure what needs to be done here) which commits need to be cherry-picked
  • cherry-pick those commits, push the new branch
  • create new PR with the title Backport {original title} and description Backport of #{PR NUMBER}
• in case of cherry-pick failure, the workflow should add comment to the original PR Unable to create backport {workflow link?}

Of course first thing to do here is to look at other OSS projects and check how are they solving stuff like this.