Add in validation of CITATION.cff file (if it exists)

[CodeGat]

See ACCESS-NRI/access-esm1.5-configs#41 (comment)

Not all *-configs-style repositories have this yet, but it seems like a good thing to add to the overall model-config-tests infrastructure.

Make it a set of steps like:

 - uses: actions/checkout@v4

 - name: Check for CITATION.cff
   id: citation-file
   run: |
     if [ -f "CITATION.cff" ]; then
       echo "exists=true" >> $GITHUB_OUTPUT
     fi
 - name: Validate CITATION.cff
   if: steps.citation-file.outputs.exist
   uses: citation-file-format/[email protected]
   with:
     args: "--validate"