Trigger pull request pipelines when the destination branch changes

Problem Definition

Please note, this request is not the same as https://jira.atlassian.com/browse/BCLOUD-17859.

This feature request is for pull request pipelines (that run based on the source branch at the moment), to also run when the destination branch of such PRs gets updated.

A pull request pipeline merges the destination branch into the working branch before it runs. If the merge fails, the pipeline stops.

If there are new commits in the source branch, the pull-request pipeline will get triggered again, doing a new merge in the "Build Setup", and then running the build.
However, if there are new commits in the destination branch of the same PR, the pipeline won't get triggered.

Suggested Solution

Considering that this pipeline runs on a merge commit, it would be desirable for some users to have the pipeline triggered when the destination branch changes as well. This way, the build will run on the new merge commit before the PR actually gets merged.