New plugin: payment via voucher (e.g. for invited speakers, co-organizers)

For our events we would like invited speakers, co-organizers, etc. to register, but we don't want to make them pay nor do we want to manually have to go in and edit their payment. Rather we'd like to give them (personalized) vouchers so that they can "pay" with that.

We'd need:

A new payment plugin that

1. Allows us to set voucher codes (general or personalized)
2. Allows users to enter the voucher codes as payment
3. Checks if the voucher code is valid (and, if personalized, if it has been used by someone else yet)

We might have some funds available to support implementation of this new plugin. Please respond to this issue if you are interested in creating this.

The issue names no files, tests, or entry points. Start by locating existing payment plugins and their voucher or payment-code validation flows; define done as supporting general and personalized codes, rejecting invalid or already-used personalized codes, and allowing valid codes to complete registration without payment.

Written by the indexing model from the issue text.