Add `jj sign` command

[julienvincent]

This is a continuation from #3007 and #2728 which adds in the jj sign command that was split out of the original PR.

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Some outstanding comments I am carrying over from #3007:

• [ ] https://github.com/martinvonz/jj/pull/3007#discussion_r1492155652
• [ ] https://github.com/martinvonz/jj/pull/3007#discussion_r1492161844
• [ ] https://github.com/martinvonz/jj/pull/3007#discussion_r1492180855

Checklist

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