feat: Add Support for Customising the Shell for Pre and Post Workflow Hooks

[X-Guardian]

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• I'd be willing to implement this feature (contributing guide)

Describe the user story

As a user of Atlantis, I would like to use shells other than sh for my pre and post workflow hooks (Bash, PowerShell etc).

Describe the solution you'd like

Have two additional, optional properties on the workflow hooks, specifying the shell and shell arguments, i.e.

repos:
- id: /.*/
  pre_workflow_hooks:
    -  description: bash test
       shell: bash
       shellArgs: -c
       run: echo $SHELL

The script will then be run using the specified shell and shellArgs, rather than the default sh -c. If the new properties are not
specified, sh -c will be used by default, so this is a non-breaking change.

Describe the drawbacks of your solution

Non identified

Describe alternatives you've considered

Non identified

[nitrocode]

Why not customize it for all steps, not just pre and post workflow hooks?

I think we have an issue for this already

[nitrocode]

Related issue #2177