Add a custom validator for "extra" field

[dinoperovic]

Is your feature request related to a problem? Please describe.
User can populate extra field with any amount of JSON data using the API. It would be useful to allow a custom validator for extra field on basket and basket item serializer, similar to how address validation works on the checkout serializer.

Describe the solution you'd like
Validate extra field before it gets saved on basket or basket item using a custom validator function configured in settings.py.

Describe alternatives you've considered
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Additional context
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