feat(publication): add shasum files to release

[davinkevin]

Hello 👋

Currently, only binaries are published, and sha digests are part of the release note. For automation purpose, it would be good to have those digests directly in the release, so we can download them using the github API, like for the original binary.

With it, we won't rely on flaky html parsing to achieve digest extraction.

PS: Of course, this is useless if https://github.com/aptible/supercronic/issues/139 is implemented.

Thank you