Add backend for pip-audit

[jyggen]

This PR adds a backend for pip-audit during the check goal.

Fixes #13770.

[thejcannon]

Can we maybe look into optional goals? So a goal only appears if you have installed a relevant plugin?

In that case I could see an audit or similar goal, which doesn't show up unless something is installed.

It's not a fast code-related-tool goal, but a heavier metadata-and-friends goal.

Audit your deps, maybe do expensive code scanning, etc...

[Eric-Arellano]

Can we maybe look into optional goals? So a goal only appears if you have installed a relevant plugin?

We have that already :)

https://github.com/pantsbuild/pants/blob/40e17902092a9b7414473f99bf32e3f3c84e5921/src/python/pants/core/goals/test.py#L310-L312

[thejcannon]