Implement jj sign and jj unsign commands

[pylbrecht]

Closes #4712

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added tests to cover my changes

[pylbrecht]

This is still WIP, but I would love to get some feedback on the general approach and if this has any design flaws that I need to address before polishing.
@yuja, @necauqua, since you have been involved in my last PR, maybe you have some suggestions?
(Of course anyone else is also welcome to leave feedback!)

// edit

Some things, which have been discussed in jj sign related issues/PRs, are still missing: showing hints if signatures have been dropped during a rebase, or builtin template(s) for displaying signatures.
I don't know if we want to make them part of this PR or address them separately.

[necauqua]

Actually left a couple of comments.

Dropped signatures hint and builtin templates can be separate PRs, yes.

Will reiterate my idea for the templates here:

There should be a some_consistent_name_for_crypt_sig template that's used in the default log template and by default it's empty.
Then there should be some_other_name template that's unused, but is present by default, with my pretty checkmarks or whatever.
And then in FAQ or in docs, there should be a tip "you can run this jj config command to set that first template to equal that second one so that you'll get checkmarks in all the default templates".

"Or you can set that template to if(signature, "[•]", "") to just differentiate signed commits from unsigned in the logs, which is very fast compared to actually verifying them"

Another thing that could be in some tips and tricks section, is an alias for jj log that has --config= set that template thing to show signatures temporarily.

Also for the builtin_log_detailed there was a change somewhere that showed a Signature: extra line - maybe do the same thing with empty default one + unused verifying one?. Or have the detailed log always verify. Maybe Yuja has an opinion there

[pylbrecht]

Wow, thanks a lot for your feedback, @necauqua and @martinvonz!
This helps a lot. Being new to the codebase (and the VCS domain), I struggle with what is expected a fair bit. It's great to receive some guidance here.

Now I am all set to return to my dungeon and work on the how.

// edit
Also, still wrapping my head around Rust, so.. please bear with me. 😅

[necauqua]

Very offtopic, sorry, but there's no chance in hell I'd have contributed to jj if is was cpp or something.
Even if it was C (that I don't dislike per se) every single project has it's own very specific patterns etc. and the build systems are plentiful.
Here in Rust we have cargo that just works, we have rustfmt, and we have the borrowchecker to not be scared of your code, and to not learn again how the particular project handles memory.

[pylbrecht]

Thanks @yuja!