Generate sha256 message digest for built binaries

[ddlees]

Relates to #306

This work generates files that contain the sha256 message digest for each binary. I'm not sure how you go about populating artifacts to the github release but would be able to do it if someone could point me in the right direction.

[fweikert]

I think the best approach right now would be to produce a single text file containing the hashes for each artifact, since we manually upload all artifacts to GitHub. We should probably add a release pipeline to Bazel CI, and might explore something like SLSA in the future.