Split spack functionality into setup and execute modules

[nick-stroud]

This change:

• Moves functionality into spack-execute module
• Adds use of bucket & startup script to store spack scripts

Tested manually:

• Actual gromacs install from cache
• Adding data files
• Chaining execute modules

Future changes:

• Rename spack install
• Migrate examples
• Clean up spack install outputs
• update documentation

Notes on review:

• Ignore readmes (only automated changes)
• Note that most of code is just moving
• Review commits independently

Submission Checklist

Please take the following actions before submitting this pull request.

• Fork your PR branch from the Toolkit "develop" branch (not main)
• Test all changes with pre-commit in a local branch #
• Confirm that "make tests" passes all tests
• Add or modify unit tests to cover code changes
• Ensure that unit test coverage remains above 80%
• Update all applicable documentation
• Follow Cloud HPC Toolkit Contribution guidelines #

[tpdownes]

A couple minor change requests.