Resolve "Add Slurm CLI plugin"

Closes #134

See merge request !95

Rename CLI options

Also, move job registration into `scord` to `slurm_spank_user_init` so
that it works both in `srun` and `sbatch`/`salloc`.

- `plugins/slurm/scord_prolog.sh`: Prolog script. Starts `scord-ctl`
   server and sets up environment.
- `plugins/slurm/scord_epilog.sh`: Epilog script. Stops `scord-ctl`
   server and cleans up environment.
- `plugins/slurm/scord_common.sh`: Common code to prolog and epilog.

Merge branch 'amiranda/119-add-a-user-level-library-for-functionalities-not-requiring-direct-scord-intervention' into 'main'

Resolve "Add a user level library for functionalities not requiring direct `scord` intervention"

This MR adds the library `libscord-user.so` with user-level interfaces to request
data transfers from `scord` from application code.

Closes #119

See merge request !103

Resolve "Tests require `scord.conf` to exist in `/etc`"

Closes #145

See merge request !102

Resolve "Update Spack"

This MR fixes an issue with conflicting CMake versions between `agio` and `scord` recipes.

It also adds a new `address` variant to enable configuring the binding address during installation with Spack.

Closes #144

See merge request !101

Add support for running tests while installing.

Resolve "Install `ping` client as utility binary"

Closes #138

See merge request !97

Fixes #138