Add idempotency key support to v4 data-mutating endpoints

When doing something that changes data (e.g. updating grants), you should be able to set an idempotency key to ensure that if the request gets run twice somehow, the operation doesn't happen twice. This can be especially helpful for endpoints that move money (e.g. transfers, issuing grants)

Start by locating the v4 data-mutating endpoints, especially those that update grants or handle transfers. Define the supported idempotency-key behavior for repeated requests and verify that each operation happens only once, including the relevant endpoint tests.

Written by the indexing model from the issue text.