generate omnibor id, similar to sha256

[ashleygwilliams]

cc @alilleybrinker

i've been volunteering with the omnibor folks and i think it'd be nice to include optionally generating omnibor artifact ids in cargo-dist. this is made quite easy by https://github.com/omnibor/omnibor-rs, which has a cli that is conveniently distributed with cargo-dist :)

a few open questions

if we want to match the sha256 impl, we'll create a file with the contents alongside each tarball. what format should the name have? i'm thinking we do <artifactname>.omnibor but i am open to what the best approach is!

which format do we want the file to contain? i'm thinking short, to match the sha256, but if we did json, that could solve the file extension issue (<artifactname>.omnibor.json) and we could potentially include other things in it.