Implement endpoint for project thumbnail

[leoll2]

Endpoint: GET /api/projects/{project_id}/thumbnail.

The endpoint should return the thumbnail of the first media of the project, so that the response is consistent over time and not affected e.g. by the addition of new items. If the dataset item is deleted, it is okay if the thumbnail changes to the next available media. Note that this endpoint is not responsible for generating thumbnails, just returning it.
If the project dataset is empty, return 404.

For reference, check the dataset item thumbnail endpoints:

training_extensions/backend/app/api/endpoints/datasets.py

Line 135 in 85ed8df

def get_dataset_item_thumbnail(