Add pre/post script support

[rstyd]

This PR addresses #773. It adds pre/post script support through a beeflow:ScriptRequirement hint which has the format:

    beeflow:ScriptRequirement:
    enabled: true
    pre_script_path: "pre_run.sh"
    post_script_path: "post_run.sh

The pre_run and post_run scripts are read using the parser and passed to the task object for the associated task. It's then read and added to the SBATCH script in the slurm runner.

[pagrubel]

@rstyd Your example works. Please add it to all the workers and can you set up a test in CI? I think there should be a way to att it to the Flux and Slurm tests using a simpler example.

[pagrubel]

As I commented it should be added to all the workers and if possible make CI tests.

[pagrubel]

1. Add pre/post to all workers.
2. Add test to slurm and flux integration test.
3. Documentation.

[kchilleri]

Same pre/post script support will be added to the flux_worker (as requested by Pat) in a separate issue/PR.
Test/documentation TBD?

[pagrubel]

@kchilleri There may be some misunderstanding. Yes the flux implementation should be separate. However, this PR is still missing documentation and integration tests. So really it shouldn't be approved as is.

[kchilleri]

@pagrubel Got it! Should I dismiss my approval?

[pagrubel]

Thanks, @rstyd All looks good. Please squash the commits and let me know when you take out your debug Asserts.

[pagrubel]

@rstyd Please squash the commits, or I can if you wish.