[backend] Allow `incomingPayments` pagination filtering by `initiatedBy`

[mkurapov]

In order to support a POS Service API for paginating across incoming payments which were initiated by the POS (RAF-1157), we should add a filter to the incomingPayments pagination backend Admin API, initiatedBy , which allows filtering incoming payments by initatedBy: CARD | OPEN_PAYMENTS | ADMIN.

For an example of how to do this, take a look at the filter key (and the OutgoingPaymentFilter GraphQL type) for the outgoingPayments pagination API.