Use `flexible_model_loader` to handle JSON/multipart-form-data in Score Set PATCH Endpoint

[bencap]

We should use the flexible_model_loader introduced in d1641de as part of the score set update PATCH endpoint to better support JSON requests to the PATCH method and improve the generated OpenAPI docs.

Using this loader would:

1. Let us support JSON requests in cases where users don't need to send file updates
2. Push JSON support into a dedicated form field, which simplifies the API docs and user submission