Add proper Libreswan support

[sergeymakinen]

Fixes #1

[codecov]

Codecov Report

Merging #2 (19a5120) into main (67ba91c) will increase coverage by 4.85%.
The diff coverage is 83.79%.

@@            Coverage Diff             @@
##             main       #2      +/-   ##
==========================================
+ Coverage   82.89%   87.74%   +4.85%     
==========================================
  Files           2        5       +3     
  Lines         380      612     +232     
==========================================
+ Hits          315      537     +222     
- Misses         50       59       +9     
- Partials       15       16       +1     

Impacted Files	Coverage Δ
exporter/metrics.go	100.00% <ø> (ø)
exporter/vici.go	28.78% <28.78%> (ø)
exporter/libreswan.go	91.66% <91.66%> (ø)
exporter/strongswan.go	96.15% <96.15%> (ø)
exporter/exporter.go	95.71% <97.87%> (+12.87%)	⬆️

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