Support Label based approver group conditions

[startnow65]

This is to support defining approver group requirement "activations" based on labels on a pull request.

With this sample config:

approvals:
  ignore: pr_opener
  minimum: 1
  pattern: ^(Approved|\+1|:\+1:|:shipit:)
  veto:
    pattern: ^(Rejected|\-1|:\-1:)
  groups:
    all_reviewers:
      from:
        orgs:
        - sample_org
      minimum: 2
    dev_only:
      from:
        users:
        - dev_1
        - dev_2
        - dev_3
        - dev_4
      minimum: 2
      conditions:
        labels:
          include:
          - dev_reviewed
    strong_reviewers:
      from:
        users:
        - strong_reviewer_1
        - strong_reviewer_2
        - strong_reviewer_3
      minimum: 1
      conditions:
        labels:
          exclude:
          - dev_reviewed

• Approver group dev_only is only activated if label dev_reviewed is present on the PR. If this label is present, then at least two members (as specified by the minimum key of the group) of the group must approve the PR, if not, this requirement is not enforced.
• Approver group strong_reviewers is only activated if label dev_reviewed is NOT present on the PR. If this label is present, then this requirement is not enforced, if not, at least one member (as specified by the minimum key of the group) of the group must approve the PR
• Approver group all_reviewers works as before