Support `sparkle:channel`

[jzau]

Thanks for the awesome project, as sparkle for osx has a channel field that we can use for beta updates only https://sparkle-project.org/documentation/publishing/#channels . It would be great if we can have the same thing.

[vslavik]

It would be great if we can have the same thing.

It certainly would, and a PR to help with it would be really appreciated. TIA!

[jzau]

I'd love to but unfortunately I'm not familiar with windows app development. I'm using this as a flutter project depedency. 😢