has_labels predicate should support looser matching

[reegnz]

Currently the has_labels predicate requires all labels to be present on the pull request.

There's some use-cases when you want to make sure it's an either/or, or one or more of the labels should be present on the pull_request, but not all of them.

Example rule I want to define:

• A pull request should have either fizzbuzz:true or fizzbuzz:false label declared
• The pull request must only have one of those labels declared.

Ideally it should be an approval rule like this:

policy:
  approval:
  - fizzbuzz-label-presence
approval_rules:
- name: fizzbuzz-label-presence
  requires:
    has_labels:
    - or:
       - "fizzbuzz:true"
       - "fizzbuzz:false"

Either make the has_labels predicate support boolean logic (like with the approvals), or provide a has_any_label predicate that allows a looser predicate matching on labels.

[reegnz]

Today the above example is achievable in a more verbose manner only:

policy:
  approval:
  - or:
    - fizzbuzz-label-true
    - fizzbuzz-label-false
approval_rules:
- name: fizzbuzz-label-true
  requires:
    has_labels:
    - "fizzbuzz:true"
- name: fizzbuzz-label-false
  requires:
    has_labels:
    - "fizzbuzz:false"

[reegnz]

Technically, now that I analyzed the problem further, or should be it's own predicate, that can nest a predicates object, eg:

policy:
  approval:
  - fizzbuzz-label-presence
approval_rules:
- name: fizzbuzz-label-presence
  requires:
    or:
    - has_labels:
      - "fizzbuzz:true"
    - has_labels:
      - "fizzbuzz:false"