Add generate_presigned_url method

[jjnesbitt]

Here's a code snippet (taken from here) that represents what's currently required to generate a presigned URL from a model using S3FileField

storage = model.blob.storage
if isinstance(storage, S3Boto3Storage):
    client = storage.connection.meta.client
    url = client.generate_presigned_url(
        'get_object',
        Params={
            'Bucket': storage.bucket_name,
            'Key': asset.blob.blob.name,
        },
    )
    return HttpResponseRedirect(url)
elif isinstance(storage, MinioStorage):
    client = storage.client if storage.base_url is None else storage.base_url_client
    bucket = storage.bucket_name
    obj = model.blob.name
    url = client.presigned_get_object(bucket, obj)
    return HttpResponseRedirect(url)
else:
    raise ValueError(f'Unknown storage {storage}')

It seems to me that everything in this code snippet is accessible by an S3FileField instance, and could be included as a method on that model. @brianhelba does this seem viable/reasonable to you?