Add more options and macOS support

[petricm]

• This extends the the action to work on macOS with the limitions to mount the repository under /User/share
• Maps all variables from Client Parameters to same name github action parameters
• All default paramers are in the action and none in the scripts
• The default.local how by default only CVMFS_CLIENT_PROFILE='signle' and CVMFS_USE_CDN='yes' as CVMFS_HTTP_PROXY is implied by CVMFS_USE_CDN

I am still having problem to configure that the testing would work for PRs but

- uses: cvmfs-contrib/github-action-cvmfs@main
+ uses: ${{ github.repository }}@main

But does not want. So there is a bit of work here still

[wdconinc]

I would keep at least one instance of the work 'cvmfs' in the name or description in action.yml. Searches for actions on the github marketplace will likely be based on that search term.

[vvolkl]

Small readme typo, otherwise looks good to me!

[petricm]

I am still working on testing... give me a bit

[petricm]

OK, have reconfigured the tests so that they run a local checkout of the code instead of the code from the main repo. But this still needs to be merged first before this will take into effect. I might have overlooked something, but I think it will be easier to spot once merged.