Check whether a specific action has been used

[chrisreddington]

I'm exploring options to determine a 'baseline' of the GitHub Actions used in workflows across a set of repositories, for example - for example, the lint step should fail if a workflow does not contain chrisreddington/my-action (regardless which version of the action is used).

There could be two different checks:

• A check which determines whether any version of the action is used
• A check which determines whether a specific version of the action is used

Then, I'd be able to use actionlint within a reusable workflow to determine whether a set of repositories conform to some baseline expectations.

[chrisreddington]

I've taken an attempt at raising a PR for this, but it's my first time deeply looking at this codebase so I suspect I'll need some changes in the PR :)

There's one bit of the implementation that I think could also use some 👀 and a second opinion in terms of how it handles reviewing multiple jobs in a workflow. Whether it should just check for existence within the workflow, or within each individual job as well... I'm not sure I have a strong opinion on which way to go right now.