Expand Actor to support UserTypes and Not

[south-mer]

What

Expand the capabilities of Actor which is used in requires, author, etc.

Why

Allow the expressing predicates such as "author must be user but not certain machine accounts" etc.

[palantirtech]

Thanks for your interest in palantir/policy-bot, @south-mer! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[south-mer]

Hello @asvoboda (apologize in advance for pinging directly because you have reviewed other PRs recently).

Could you please take a look and let us know what you think about potentially merging this PR? Thank you.

[asvoboda]

I think it would be useful to describe in more detail what problems you are having first and if they can be expressed using current policy primitives.

[south-mer]

@asvoboda thank you for your question.

We are writing rules that specifically target pull requests with authors/contributors that are NHIs like GitHub Apps and machine users. For the prior, we would like to use the UserTypes, while for the latter one we would like to use Not.

We are also writing some specific rules that further re-include some GitHub Apps, resulting in nested Nots, in order to smooth the onboarding of repositories with existing automation onto policy-bot.