add a CI test to check submodule commit hash consistency

[alperaltuntas]

Add a test module to check if the MOM6 submodule commit is the same as what's specificed in .gitmodules

[alperaltuntas]

@mnlevy1981 Need a few corrections here. Will let you know.

[mnlevy1981]

It might be easier to mimic ESCOMP/CESM#317 -- use git fleximod to check out all the externals, and then run git diff --exit-code; I guess that will require either adding git fleximod as an external or checking it out as part of the action.

[alperaltuntas]

This PR is now ready, but happy to close this in favor of ESCOMP/CESM#317

[mnlevy1981]

I think it makes sense to have a test in MOM_interface even if CESM has a similar test for all components, but this looks like a complicated solution that will only look at MOM6 while git diff --exit-code will also catch issues with inconsistent tags of MARBL and stochastic_physics; the catch is that I don't think we carry git fleximod around in this repo so we'd either need to add it or the github action would need to check it out

[alperaltuntas]

New PR, with no overengineering: #229