feat: Prepare OpenVPN server plugin

[jkroepke]

What this PR does / why we need it:

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

Particularly user-facing changes:

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• The PR title has a summary of the changes
• The PR body has a summary to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

Attention: 70 lines in your changes are missing coverage. Please review.

Comparison is base (52ccc49) 71.16% compared to head (40f5e3d) 71.78%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #40      +/-   ##
==========================================
+ Coverage   71.16%   71.78%   +0.62%     
==========================================
  Files          17       19       +2     
  Lines        1134     1276     +142     
==========================================
+ Hits          807      916     +109     
- Misses        274      315      +41     
+ Partials       53       45       -8     

Files	Coverage Δ
cmd/root.go	70.99% <100.00%> (+1.10%)	⬆️
internal/config/config.go	96.09% <100.00%> (+6.90%)	⬆️
internal/oauth2/provider.go	78.64% <100.00%> (ø)
internal/openvpn/client.go	92.20% <100.00%> (+0.54%)	⬆️
internal/state/state.go	84.61% <100.00%> (-0.39%)	⬇️
internal/oauth2/handler.go	54.19% <76.19%> (+4.78%)	⬆️
internal/openvpn/callbacks.go	0.00% <0.00%> (ø)
internal/config/load.go	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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