Add compatibility with Pydantic v2

Updated all Pydantic model classes to be compatible with both pydantic v1 and v2. To achieve this we use the pattern

try:
    from pydantic.v1 import Something
except ImportError:
    from pydantic import Something

This should be compatible with all pydantic versions back to 1.7.4. The custom json_encoder has also been updated to handle both v1 and v2 model classes.