Spackage with Plugin support

[rbberger]

PR Summary

This updates the Singularity-EOS spackage to support plugins. It introduces a new directive singularity_eos_plugin which is used to register a plugin with a relative plugin path. While convenient, we still have to see if spack upstream would accept such a spackage change.

The plugins variant can then choose one or more of these plugins to be used. Unknown plugin names will result in an error.

variant can be used to set the variant header location.

PR Checklist

• Adds a test for any bugs fixed. Adds tests for new features.
• Format your changes by using the make format command after configuring with cmake.
• Document any new features, update documentation for changes made.
• Make sure the copyright notice on any files you modified is up to date.
• After creating a pull request, note it in the CHANGELOG.md file.
• LANL employees: make sure tests pass both on the github CI and on the Darwin CI

If preparing for a new release, in addition please check the following:

• Update the version in cmake.
• Move the changes in the CHANGELOG.md file under a new header for the new release, and reset the categories.

[Yurlungur]

LGTM but @mauneyc-LANL should probably also take a look

[Yurlungur]

@rbberger one more minor comment. Then from my perspective this is good to g. Since @mauneyc-LANL also agrees, go ahead and pull the trigger when you're ready. (Or ask me or @dholladay00 to do so.)