More machine-readable donation options

[darkdragon-001]

Something like https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/displaying-a-sponsor-button-in-your-repository

Such that app stores can more easily offer richer donation options.

The current url type donation is often implemented to only allow a single option. Also in order to find out which service, one needs to parse the URL again.

Maybe something like <donation type="opencollective">slug</donation>?