Executor/pdu: Add support for SNMPv3 / credentials

Right now, we are using SNMPv1 by default on SNMP-enabled PDUs, which does not authenticate requests.

Let's keep that as a default, and add support for specifying the SNMP version, username, and password as config parameters.

See https://easysnmp.readthedocs.io/en/latest/session_api.html for more information about what we may want to allow configuring.