Systemd timer format for periodic jobs

Hi there, I want to suggest implement PeriodicSpecSystemd

### Proposal
I want to add another one PeriodicSpecType.
https://www.freedesktop.org/software/systemd/man/systemd.time.html
It is more flexible and usefull for many system administration who knows how to managing systemd.

I forked you cronexpr library, and add systemd timers functionality. (also rewrite some tests and add my)
https://github.com/WinnerSoftLab/cronexpr

I can implement it in Nomad, but after my PR https://github.com/hashicorp/nomad/pull/17858 will be merged , because it will have a lot conflicts if I violate the order merges.

### Use-cases
Nomad get more flexible and famous format for describe periodic job times.

Start by reviewing the proposed PeriodicSpecSystemd type and the systemd.time format linked in the issue, then compare the referenced cronexpr fork and its added tests. Done means Nomad accepts systemd timer expressions as another periodic specification type, with coverage for the supported behavior.

Written by the indexing model from the issue text.