Upload a list of files to be indexed. Optionally upload them to an Airbyte
Destination S3 bucket if the feature flag is enabled.

Usage:
Use the following curl command to upload files:

curl           -F files=@app/files/arya_stark.txt           -F files=@app/files/eddard_stark.txt           -X POST <host>/file-upload

Returns:
dict: A dictionary containing the results of the upload operation.
Example:
{"results": {"documents_written": <num_documents_written>}}