add a github action to compare the XML outputs between the base branch and the PR branch

[guoqing-noaa]

This PR adds a Github Action that automatically generates a set of Rocoto XML files for different example exp files and different configurations.

It serves as a sanity check to ensure that existing capabilities and dependencies continue to function correctly when changes are made to the setup_rocoto.py process, helping to minimize the need for hotfixes (e.g., #1431).

In some cases, differences in the generated XML files are expected—for example, due to version bumps, new features, or other enhancements. In these situations, the GitHub Action will initially fail.

Developers should then manually review and verify the differences. Once all changes are confirmed to be correct, the PR author can update tests/.ignore.compare_xml_outputs by adding the corresponding PR number to the first line of the file. This signals the GitHub Action to ignore the known differences and pass successfully.

This github action only launches when there are any changes to the setup_rocoto.py process.

[guoqing-noaa]

The behavior of this github action can be previewed at the following testing runs:

How it fails:

https://github.com/guoqing-noaa/rrfs-workflow/actions/runs/24179498356/job/70568612270

How the "ignore" works:

https://github.com/guoqing-noaa/rrfs-workflow/actions/runs/24194035253/job/70618846179?pr=12

[guoqing-noaa]

Tested and worked as expected. Ready for review and merge. Thanks!