Add OR logic to `required_labels`

[cgwalters]

Followup to #100

Basically I'd like to switch to org.opencontainers.image.version and drop the legacy version, but the policy won't allow me to do that.

I think the code here needs to be extended to support a rule that one of a set of labels is present.

  required_label_sets:
  - name:
      - org.opencontainers.image.version
      - version
    description: Version number, see https://github.com/opencontainers/image-spec/blob/06e6b47e2ef69021d9f9bf2cfa5fe43a7e010c81/annotations.md?plain=1#L27

or so