Fix dependency on pillow in libs
Both libs (libcommon and libapi) use the pillow package in their code, but do not explicitly depend on it.

See:
- In libcommon src code:
https://github.com/huggingface/datasets-server/blob/33f0c9c098d1fb94ad09c8b6e9f89cebf08d8f20/libs/libcommon/src/libcommon/viewer_utils/asset.py#L10
- In libapi tests code:
https://github.com/huggingface/datasets-server/blob/33f0c9c098d1fb94ad09c8b6e9f89cebf08d8f20/libs/libapi/tests/test_response.py#L10

This PR adds their explicit dependence on pillow.
Fix dependency on pillow in libs: Both libs (libcommon and libapi) use the pillow package in their code, but do not explicitly depend on it.

See:
- In libcommon src code:
https://github.com/huggingface/datasets-server/blob/33f0c9c098d1fb94ad09c8b6e9f89cebf08d8f20/libs/libcommon/src/libcommon/viewer_utils/asset.py#L10
- In libapi tests code:
https://github.com/huggingface/datasets-server/blob/33f0c9c098d1fb94ad09c8b6e9f89cebf08d8f20/libs/libapi/tests/test_response.py#L10

This PR adds their explicit dependence on pillow.