code: func (service *GCSService) ComposeFrom(ctx context.Context, objSrcs []*storage.ObjectHandle, dstParams GCSObjectParams, contentType string) (uint32, error) {
  	dstObj := service.Client.Bucket(dstParams.Bucket).Object(dstParams.ID)
  	c := dstObj.ComposerFrom(objSrcs...)
  	c.ContentType = contentType
  	_, err := c.Run(ctx)
  	if err != nil {
  		return 0, err
  	}
  
  	dstAttrs, err := dstObj.Attrs(ctx)
  	if err != nil {
  		return 0, err
  	}
  
  	return dstAttrs.CRC32C, nil
  }
docs: // ComposeFrom composes multiple object types together,