feat: automatic TLS certificate reload

[flavio]

When running on Linux, monitor changes done to the TLS certificate and key used by the https server.

When both change, react by updating the TLS configuration.

This allows to rotate the TLS certificate used by a Policy Server without having to restart the process.

Notes:

• This is not covered by the unit tests right now. I don't know yet how hard it would be to test it. I'll leave a comment here once I know more about the topic
• This change is part of the upcoming work to handle certificate rotation inside of the whole KW stack
• Ideally, we can merge this code right now and make it part of the 1.15 release since it won't cause any harm

[codecov]

Codecov Report

Attention: Patch coverage is 70.45455% with 13 lines in your changes missing coverage. Please review.

Project coverage is 61.44%. Comparing base (1677586) to head (5e94a62).
Report is 3 commits behind head on main.

Files	Patch %	Lines
src/lib.rs	70.45%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #826      +/-   ##
==========================================
+ Coverage   58.45%   61.44%   +2.99%     
==========================================
  Files          17       17              
  Lines         994     1035      +41     
==========================================
+ Hits          581      636      +55     
+ Misses        413      399      -14     

Flag	Coverage Δ
integration-tests	53.86% <70.45%> (+3.31%)	⬆️
unit-tests	33.20% <0.00%> (-1.41%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[fabriziosestito]

Looks great! As far as testing goes, we could write an integration test. However, I don't see this as critical and will be fine leaving this part untested.

[flavio]

UPDATE: all done

I've updated the code to have an integration test. The integration test will fail until I do the following changes:

• merge fix: ensure consumers of policy-fetcher can compile policy-fetcher#201 and tag a new release of policy-fetcher
• tag a new release of policy-evaluator that consumes this version of policy-fetcher

[viccuad]

LGTM! 🚀