Enable App Icon upload

[inigobasterre]

I've been creating apps using the python-slack-sdk together with the apps.manifest.create Web API method, but I have not been able to programmatically add an icon usiong /assets/*.png, like the Deno SDK does.

[cchensh]

Hi @inigobasterre , thank you for reaching out, currently we are working on a new API endpoint to support app icon uploading programmatically, will keep you posted once it's ready to use.