Automate checksum file generation for release tarballs

For each release we upload the linux and darwin tarballs as assets, along with checksum files for them.
Add a script/make target similar to one from [build-tooling repo](https://github.com/aws/eks-anywhere-build-tooling/blob/main/build/lib/update_checksums.sh) to generate these checksum files for the release assets

Start by inspecting the repository's release process and the checksum-generation example in build-tooling's update_checksums.sh. Add the requested script or make target so the Linux and Darwin release tarballs each produce checksum files, then verify the generated files alongside the release assets.

Written by the indexing model from the issue text.