feat: post workflow hooks

[tim775]

This PR implement functionality to run user defined scripts after all workflows have executed. This is a complement to Pre Workflow Hooks that addresses #1954 and builds off previous work done by @gezb.

As with pre_workflow_hooks, the server's repo config can contain custom post_workflow_hooks scripts:

- id: your-repo
   post_workflow_hooks:
   - run: ./my-script
   - run: |
        my bash script inline

Our specific use case is to generate a combined cost estimate report that contains the cost of all workflows. So with a server config like

# repos.yaml
workflows:
  myworkflow:
    plan:
      steps:
      - init
      - plan
      - run: infracost breakdown --path=$PLANFILE --format=json --out-file=/tmp/$BASE_REPO_OWNER-$BASE_REPO_NAME-$PULL_NUM-$WORKSPACE-$REPO_REL_DIR-infracost.json
repos:
  - id: /.*/
    workflow: myworkflow
    post_workflow_hooks:
      - run: infracost output --path=/tmp/$BASE_REPO_OWNER-$BASE_REPO_NAME-$PULL_NUM-*-infracost.json --format=github-comment --out-file=/tmp/infracost-comment.md
      # Now report the output as desired, e.g. post to GitHub as a comment.
      ...

A separate cost estimate comment is posted on the PR (sort of like CodeCov comments but for cost):

[chenrui333]

Thanks @tim775!

[alikhajeh1]

@nishkrishnan, @jamengual and @chenrui333 I just wanted to say thank you for shipping this as it simplified the Infracost integration that we shipped today. It enabled 1 cost estimate comment to be shown for multiple Terraform dirs/workspaces 🎉

[jamengual]

I'm glad it worked out, thanks for the contribution.