Add storing systemd version in stats struct

[cooperlees]

• Add the version into the main MonitordStats stats struct
• Create a dedicated struct to parse out the version string
• Add Display + from string traits

Test

• Update unit tests
• Add tests for new struct parsing
• Run on my fedora 40 box

  cooper@cooper-fedora-MJ0J8MTZ:~/repos/monitord$ cargo run -- -c monitord.conf
  Compiling monitord v0.8.4 (/home/cooper/repos/monitord)
  Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.11s
   Running `target/debug/monitord -c monitord.conf`
  I0618 16:04:28.082939 829653 src/main.rs:28] monitord: Know how happy your systemd is! 😊
  I0618 16:04:28.083237 829653 src/lib.rs:114] Starting stat collection run
  E0618 16:04:28.085213 829653 src/networkd.rs:306] Unable to get interface links via DBUS - is networkd running?: D-Bus error: Could not activate remote peer 'org.freedesktop.network1': activation request failed: unknown unit (org.freedesktop.DBus.Error.NameHasNoOwner)
  I0618 16:04:28.101066 829653 src/lib.rs:176] stat collection run took 17ms
  {
  ...
  "monitord.units.total_units": 680,
  "monitord.version": "255.7-1.fc40"
  }