Add service file for systemd

[Callisto13]

As part of my docs writing I realised that having an option to manage flintlockd with systemd looks a bit more pro than "start the server with an &".

The --parent-iface flag is of course required and means some sort of user input. I did consider an EnvironmentFile, but it's a bit "eh" and the systemd creators deeply regret its inclusion.
I am documenting that users need to do something here.

[codecov-commenter]

Codecov Report

Merging #324 (06e753f) into main (5f8d70c) will not change coverage.
The diff coverage is n/a.

❗ Current head 06e753f differs from pull request most recent head c937ad8. Consider uploading reports for the commit c937ad8 to get more accurate results

@@           Coverage Diff           @@
##             main     #324   +/-   ##
=======================================
  Coverage   58.03%   58.03%           
=======================================
  Files          51       51           
  Lines        2509     2509           
=======================================
  Hits         1456     1456           
- Misses        936      937    +1     
+ Partials      117      116    -1     

Impacted Files	Coverage Δ
infrastructure/containerd/event_service.go	56.14% <0.00%> (-3.51%)	⬇️
pkg/queue/queue.go	100.00% <0.00%> (+6.06%)	⬆️

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[yitsushi]

We have viper included, i wonder why we don't have a --config option.

[Callisto13]

are we not removing viper?

[Callisto13]

but yeh if urfave/cli has a config file then we can use that

[richardcase]

but yeh if urfave/cli has a config file then we can use that

The PR that Soule has done for urfave/cli does have a config file option.

[Callisto13]

👍 do we want that as part of this PR or can i move forward with this one and come back to that?

[richardcase]

+1 do we want that as part of this PR or can i move forward with this one and come back to that?

Lets come back to it