Request: Enable Harness OPA Policy Sets to select the Git branch used for policy evaluation and enforcement, instead of always using the repository's default (main) branch.

Why: Allow teams to test and validate policy changes from a feature branch in an active environment before merging to main.

Expected Behavior: When a policy set is configured to use a different branch (for example, feature/new-policy), Harness should load, evaluate, and enforce the OPA policies from that branch. Switching branches should immediately change the source of truth for policy enforcement, allowing easy testing and promotion of policy changes without impacting the default branch.