Add GitHub workflow: Check that all files in our namelists are on GDEX server

I'm imagining a script that would check all files in:
- `cime_config/**/user_nl_clm`
- `bld/namelist_files/namelist_defaults*.xml`

There's Python code to do that [here](https://github.com/ESMCI/inputdataTools/blob/f9a9bb807d004084d3a693fd6ab79bde64261f0e/rimport#L290-L319) that we could copy.