_frontend/widget: Allow showing the CAS digest of built artifacts

[Kekun]

This adds the artifact-cas-digest format string to the show command.

This is a first step implementing #1984.

• Add tests, e.g. building an artifact and comparing the printed digest

[Kekun]

I added a test, it's probably not enough though…

[gtristan]

I feel that cas-digest is too generic. This is actually the cas digest of the built artifact, we could (maybe not now, but in the future) have cas digests of other things (like the source code, or the buildtree if we have one)

I tend to agree, while the “CAS” is an implementation detail it’s fairly well known, I think I’d rather avoid exposing the word in APIs if we don’t already have it.

Also valid point about source caches.

let’s use another name, perhaps content-hash ? Maybe that doesn’t capture the implication that it is the artifact content… maybe output-hash or artifact-content-hash ?

Also, was this man page updated using the automated generation ? We should only update the command description in cli.py and use the python command to automatically update all the man pages and commit that as a separate commit… that command is documented… somewhere in our HACKING file(s)….

[Kekun]

Isn't "hash" more technical than "digest" too (genuine question, I don't know)? What about just artifact-digest (or artifact-hash if you prefer)? Any reason to prefer "content"? I'll let you two decide the right words to use as you are the BuildStream specialists. :)

The man page what updated manually, I'll update it in a different commit then.