Add pydantic type for validating and deserializing S3FF values

[zachmullen]

This library currently provides the s3_file_field.rest_framework.S3FileSerializerField for downstreams to use with Django Rest Framework. However, some downstreams are now using pydantic instead of DRF serializers, e.g. when using django-ninja for REST APIs.

This PR provides the pydantic equivalent serialization helper, to be used by downstreams as follows:

from pydantic import BaseModel
from s3_file_field.pydantic import S3FileFieldValue

class MyModel(BaseModel):
    my_file_field: S3FileFieldValue

When instantiated, this model's my_file_field value will be validated and converted such that it can be passed directly into an S3FileField Django model field.