Feature request: Incorporate security checks into an agent command

There's some great guidance in https://buildkite.com/docs/agent/v3/securing#strict-checks-using-agent-hooks for adding security checks to hooks.

However, we've found this ends up being difficult when considering all the checks one might want.

Would be great if the agent could support:

- checking plugins against an allow list
- checking the repository against an allow list
- checking the author against an allow list

Something like:
```
build-agent security-check
--plugin-allow-list /something/plugins.txt \
--repository-allow-list /something/repositories.txt \
--author-allow-list /something/authors.txt
```

Could then even get fancier with strict version checks etc