feat: add fine-grained pre/post commands to terragrunt workflows

[chris11-taylor-nttd]

This adds support for arbitrary commands to be run before and after all Terragrunt plan/deploy/destroy steps. Additionally, for workflows that contain multiple steps (e.g. plan and then deploy), adds a "shared" commands structure that will apply to both for convenience.

This opens the door for repo-specific commands to be run around the core Terragrunt workflows. One immediate need is around an internal tool utilizing our Azure web_app module, which has a lifecycle rule that prevents changes to app_settings. This is done because there are other services and features within Azure that will dynamically update these configuration items, which Terraform detects as drift and then attempts to correct on next run. For setups utilizing this module, an external script to additively apply expected configurations, rather than a full sync, can be used to ensure that configurations are published through the pipeline.

Also contains a few minor formatting/indentation changes. Evidently I didn't have format-on-save turned on the last time I messed with these files.