Add 'asset_label' support

[yookoala]

Support customized asset label (optional). Allow user to have a human readable asset name other than the filename itself.

Maps to the optional label parameter of the API: https://docs.github.com/en/rest/reference/repos#upload-a-release-asset

If an asset_label is not provided, the API request will not have the label parameter. The change is therefore backward-compatible.