Provide entity id to custom tokens for non-contact entities

(I thought there'd be an issue for this already but couldn't find one. Is this a duplicate?)

For hook-based custom tokens for non-contact entities, the hook_civiccrm_tokenValues implementation is given the relevant contact ID, but not an ID for the relevant non-contact entity, e.g., the relevant contribution for a thank-you letter.

As a result, the custom token can return aggregate or other generalized information about the contact's entities (e.g., last contribution, total contributions in a fixed date range, etc.) but can't return anything based on the specific entity (e.g. contribution).

This could potentially be resolved by finding a way to provide the relevant entity ID to hook_civiccrm_tokenValues.