chore: Add systemd user units for atuin daemon

[Flowdalic]

Now that https://github.com/atuinsh/atuin/pull/2171 is merged, we can add sensible systemd user units for atuin daemon.

Checks

• [x] I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
• [x] I have checked that there are no existing pull requests for the same thing

[ellie]

Hey! Thanks for the PR, but we currently lack these intentionally (and never because of #2171, which has always had several trivial workarounds)

The daemon is experimental, and initially prompted as a workaround for #952, which is caused by a bug in OpenZFS. We do not yet prompt people to use it, and do not yet package a way for this to be easily setup. Our docs highlight that it is experimental, and also that we do not yet provide things like systemd units.

We've discussed methods of doing so here: https://forum.atuin.sh/t/moving-atuin-to-a-daemon/78, which also includes some unit files people are choosing to use.

Until we're ready to support wider use of the daemon, we won't be including ways for people to execute it. If package maintainers choose to then they can, however they will be responsible for supporting it.

One concern is in ensuring that the daemonized process stays up to date - a supported systemd unit would probably require some sort of watcher there too.