[Feature Request] Show backport conflicts pre-merge on PRs

[andrross]

Is your feature request related to a problem? Please describe

The backport automation, when it works, greatly reduces the manual effort to maintain other branches. However, sometimes it fails but we don't get any indication that it will fail until after the PR on main is committed. I think it would be nice to know ahead of time if my PR will generate a conflict on backport, and to know which files will conflict. This gives me an opportunity to fix it ahead of time if possible. (The usual avoidable conflicts are due to an incorrect CHANGELOG entry, but this still strikes me as being potentially useful even if we improve the CHANGELOG process to avoid contention on a single file)

Describe the solution you'd like

If I add a backport 2.x label to my PR, then a GitHub action is run that tells me either one of the following:

• My commit will cleanly backport to the 2.x branch
• My commit will not cleanly backport, and which files have conflicts that must be manually resolved

Related component

Build

[peternied]

[Triage - attendees 1 2 3 4 5 6 7 8]
@andrross Thanks for filing this issue, this would be useful for contributors authoring pull requests