journald logging support tracking issue

[jabedude]

@lucab if you're ok with it, I'd like to add support for logging to the system journal, similar to the go package.

[lucab]

I'll be super-happy to review that, yes.

A few general notes as a quick braindump:

• the Go implementation uses a globally cached connection for historical reasons. If you need to cache something here too, feel free to have a struct (e.g. JournalConnection) holding it instead.
• for temporary FD creation, there is memfd_create in nix.
• send function can be implemented either in async or blocking way. We can simply focus on the latter for now, as I am not sure all the pieces are in places in other libraries to implement the former.
• for an idea of how consumers would use journal-send logic, see https://github.com/slog-rs/journald/blob/master/src/lib.rs