Support including checksums for the GitHub Release created by gh with:

gh release create --checksums

This should generate a .txt file with the checksums of all the files included in the release, following the goreleaser strategy (so we at least follow a pattern), the same one used in the gh official releases as well.

This option should be enabled by default generating checksums using SHA256 in order to spread best practices, but users can optionally toggle it on or off.