fix(upload-client/multipart): set custom chunk size for multipart

[optlsnd]

If a multipart upload is started with a custom part size, the part size must be set in the request to /multipart/start/. Otherwise, the default 5 MB part size is used, which causes the subsequent request to /multipart/complete/ to fail:

{
    "error":
    {
        "status_code": 400,
        "content": "Can not complete upload. Wrong parts size?",
        "error_code": "MultipartFileCompletionFailedError"
    }
}