Use either from_iso_format in DatetimeParameter or timestamp() but not both in

[lfse-slafleur]

Currently DatetimeParameter in workflow_types.py uses the ISO format representation for datetimes in the functions from_json_config, from_pb_message and to_pb_message while the functions to_pb_value and from_pb_value use the unix timestamp with datetime.timestamp() function. Both sets of functions convert the datetime into something that is protobuf compatible (albeit that {from,to}_pb_message() functions use a protobuf Struct`) so they should be using the same representation. Right now using different representations is unnecessary and confusing.