OPA policy input: include inline review comments

[dlorenc]

Overview

Follow-on to #93. Once inline review comments are implemented, make them available in OPA policy input so policies can require inline approval of specific paths before merge.

Scope

Define the shape of comment data in the OPA input document passed to policy evaluation at merge time. Likely under input.comments alongside the existing input.reviews and input.check_runs.

Open questions

• Should comments be filtered to the current `head_sequence` only, or include all comments on the branch?
• Should the policy be able to require that specific paths have at least one comment (e.g. OWNERS files)?
• Should `review_id`-linked comments be grouped differently from standalone comments?

Dependencies

• Inline review comments on files #93 must be merged first