Refactor CustomDateField

[ameyarao98]

The current implementation is a bit hacky, and would not work with pydantic v2. It could be made into a datetime object, which Pydantic can parse from various formats,making it easier to create a message if not using a string. There should still however be a manual check when parsed from a string that it fits the ISO standard. Some custom logic (or someting in dateutil?) may be needed to turn the datetime into the proper format

[fubuloubu]

I have been using this w/ Pydantic V2:

def iso_format(dt: datetime) -> str:
    return dt.isoformat()

UTCTimestamp = Annotated[
    datetime,
    PlainSerializer(iso_format, return_type=str),
]

You can add a PlainValidator for help in converting to the proper iso string

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Edit: You might also have to add WithJsonSchema to the annotation to get it to work well with OpenAPI schema generation