DevOps: Add formatter check to pr_checks_backend workflow

[zackdreitzler]

Describe the task

Add a formatter check to any PR impacting the data directory and its sub directories. This check ensures that new python code is formatted using the black formatter.

Acceptance Criteria

• A format checking step exists on the pr_checks_backend workflow
• When all files report as formatted, report a success to the PR that triggered this action
• When one or more files report as not formatted, report a failure to the PR that triggered this action

Additional context

This story is currently blocked until #638 is completed.

[Michael-Dratch]

Can I pick up this task?

[nlebovits]

@Michael-Dratch I actually double-dipped and folded this one into my PR #756. It should run build checks for both docker containers, a formatter check with black, and linting with flake8. Do you mind reviewing the PR?

[nlebovits]

closed by #756