Uploads a single part of a large file in a multipart upload session and returns the ETag and part number.
This endpoint is used as part of the multipart upload workflow:

1. First, initialize a multipart upload using InitiateMultipartUploadAsync
2. Upload individual parts using this endpoint
3. Complete the upload using CompleteMultipartUploadAsync

The part number must be between 1 and 10,000, and parts must be uploaded sequentially.
The same part number can be reused to replace a previously uploaded part.