Reserve a repo signing key, for use with account creation.

*This endpoint is part of the atproto PDS server and account management APIs. Requests often require authentication and are made directly to the user's own PDS instance.* *To learn more about calling atproto API endpoints like this one, see the [API Hosts and Auth](/docs/advanced-guides/api-directory) guide.* Reserve a repo signing key, for use with account creation. Necessary so that a DID PLC update operation can be constructed during an account migraiton. Public and does not require auth...