feat(connectivity): Allow internet connection validation prior to endpoint execution

[TwiN]

Summary

While Gatus is used to monitor other services, it is possible for Gatus itself to lose connectivity to the internet.
In order to prevent Gatus from reporting endpoints as unhealthy when Gatus itself is unhealthy, you may configure
Gatus to periodically check for internet connectivity.

All endpoint executions are skipped while the connectivity checker deems connectivity to be down.

Checklist

• Tested and/or added tests to validate that the changes work as intended, if applicable.
• Updated documentation in README.md, if applicable.

[codecov-commenter]

Codecov Report

Patch coverage: 76.92% and project coverage change: +0.01 🎉

Comparison is base (6908199) 81.97% compared to head (0248ce8) 81.99%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #461      +/-   ##
==========================================
+ Coverage   81.97%   81.99%   +0.01%     
==========================================
  Files          56       57       +1     
  Lines        4261     4287      +26     
==========================================
+ Hits         3493     3515      +22     
- Misses        600      602       +2     
- Partials      168      170       +2     

Impacted Files	Coverage Δ
config/config.go	74.89% <25.00%> (-1.68%)	⬇️
config/connectivity/connectivity.go	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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