Run arbitrary expressions as a job

[roberth]

Add at least

• a new hci subcommand to create a new job for Hercules CI based on local expressions
• differentiation in the dashboard to separate this from git-based jobs

Related: hercules-ci/hercules-ci-agent#309

[nomeata]

Especially for the use case of invoking it from a GitHub action, the ability to pass simple metadata (a custom job title, maybe a link that would show up in the dashboard and could point to the commit or action job) would be nice

[nomeata]

Maybe simpler, but helpful in cases I care about: a command hci wait that I can run in an GitHub action to wait for Hercules CI to build stuff (the whole jobx or maybe just one attribute), before running nix-build in GHA and doing further deploy-like actions. (Hercules CI has effects too, but some things might simply be easier from an GitHub action.)