Spack DAG Builder GitHub Action

[vsoch]

We would like to develop a spack GitHub action that can test an updated package, and then generate a DAG via spack ci that can be piped into GitHub Actions to test all related builds. This will help a developer answer the question "Will my change to this package break anything else?" and can be used separately from production spack, as a way to pre-test a package and possibly save cycles on production spack CI. Dependencies for this work include:

• creating a general function for spack ci to generate a data structure with metadata that can be used here (or for snakemake, as I was testing)
• having spack ci updated to then output a matrix for GitHub actions
• adding additional functionality to be more efficient (e.g., build caching, logs, similar)

c.c. @alecbcs and @scottwittenburg