RCODE filter should support string values and multiple values

[rboucher-me]

The DNS handler filter for RCODEs (only_rcode) should support string values (on top of the current support for numerical values). It should also accept a list of items. This would make it consistent with the QTYPE filter behavior (only_qtype).

currently:
only_rcode: int

should become:
only_rcode: str[]