Alpha/DAL: prepare to validate Dal_entrapment_evidence

What

Prepare for the validation of the DAL entrapment evidence operation.

On top of !15959 (merged).

Actual validation MR: !15836 (merged)

Why

Part of %DAL: baker incentives (part 2: accusations).

Manually testing the MR

Not yet. However, it's under a feature flag.

Checklist

• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).