Finalize the multipart upload of a custom thumbnail

Commits the multipart upload of a custom thumbnail. Background processing then generates the sized `customThumbnail` files and registers them on the asset. Unlike `/v1/assets/complete-upload`, this does not register an original file on the asset and does not trigger media transcoding.

curl -X POST "https://example.com/assets/497f6eca-6276-4993-bfeb-53cbbbba6f08/custom-thumbnail/complete-upload" \  -H "Content-Type: application/json" \  -d '{    "key": "string",    "uploadId": "string",    "parts": [      {        "ETag": "string",        "PartNumber": 1      }    ]  }'

{  "status": "completed",  "key": "string"}