Upload checksum alongside release file

[moritzheiber]

I would love this Action to support uploading either a checksum in a single file, for each file or in an aggregated file alongside the other release artifacts. This is going along the lines of the checksum feature of goreleaser.

[svenstaro]

Fancy attempting to implement this? I just did a large refactor with proper E2E testing and now it should be easier to contribute.