[Reimbursement] Add Disbursement as payout method (only accessible by admin)

Creating this issue as RFC:

Allowing Disbursement as payout method (only accessible by admin). When an admin approves, they can choose to discard the existing payout method and process it as a disbursement instead.

I know it'd be useful for HQ organizations. But do we want to implement this?

Start by reviewing the repository's reimbursement approval, payout-method, and disbursement entry points; the issue names no files or tests. First determine whether admin-only disbursement processing is wanted, then define the approval flow, replacement of the existing payout method, and the tests needed to establish completion.

Written by the indexing model from the issue text.