Fix TIPydantic serialization of MappedOperator

[dstandish]

Previously we relied on SerializedBaseOperator.serialize_operator for serialization of all Operator objects but this is no bueno because it does not work for mapped operator. Instead we use BaseSerialization.serialize, which calls the right method for each obj type. To make this actually work for db isolation though, we had to do a few more things. 1. operator_class obj was not deserialized properly so fixed that. It's a weird case cus it roundtrips to a dict -- not a class. Also expand_input did not work quite right because it roundtrips to _ExpandInputRef. Then we had to make a small change to get_map_type_key since we might have _ExpandInputRef here.