ci: add Spack installation and test workflow (closes #235)

[TheAssembler1]

Related Issues / Pull Requests

Closes #235

Description

Adds a GitHub Actions workflow (.github/workflows/spack-install.yml) that
automatically tests the Spack installation of PDC on every PR. The workflow
uses Spack's binary build cache to install all dependencies (Mercury,
libfabric, MPI, etc.) and then builds PDC from the PR's source tree using
CMake with BUILD_TESTING=ON. Finally it runs the serial CTest suite.

What changes are proposed in this pull request?

• New feature (non-breaking change which adds functionality)

Checklist:

• My code modifies existing public API, or introduces new public API, and I updated or wrote docstrings
• I have commented my code
• My code requires documentation updates, and I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes