Passing attributes through during label coercions

[brimoor]

Resolves #1989.

Previously dynamic attributes were not included when performing coerced exports, such as exporting image patches defined by a Detections field in a Classification export format. Now, dynamic attributes are included in these cases.

Example usage:

import fiftyone as fo
import fiftyone.zoo as foz

dataset = foz.load_zoo_dataset("quickstart")

dataset.limit(10).export(
    export_dir="/tmp/quickstart-patches",
    dataset_type=fo.types.FiftyOneImageClassificationDataset,
    label_field="ground_truth",
    include_attributes=True,
)

python -m json.tool /tmp/quickstart-patches/labels.json

...
        "000061": {
            "label": "traffic light",
            "attributes": {
                "iscrowd": 0.0,
                "area": 47.59534999999962
            }
        },
        "000062": {
            "label": "person",
            "attributes": {
                "iscrowd": 0.0,
                "area": 794.5827999999998
            }
        },
...

[ehofesmann]

LGTM!