Feature request: support fdstore functionality

[happy-dude]

Hey team,

I was wondering if it was possible to add support for the fdstore interface with this library?

From the documentation,

TL;DR: The systemd service manager may optionally maintain a set of file descriptors for each service. Those file descriptors are under control of the service. Storing file descriptors in the manager makes is easier to restart services without dropping connections or losing state.

Currently, go-systemd has the ability to retrieve file descriptors stored in this manner via Files() (https://pkg.go.dev/github.com/coreos/go-systemd/v22/activation#Files), but does not have the ability to set/upload the descriptors (via sd_pid_notify_with_fds?)

If this is desirable functionality, I can look into submitting a patch/PR with some guidance and prior art to reference!