feat(monitor): support healthchecks.io

[favonia]

Close #93.

[codecov]

Codecov Report

Merging #160 (8fe7dd0) into main (b3fc809) will decrease coverage by 0.03%.
The diff coverage is 63.28%.

❗ Current head 8fe7dd0 differs from pull request most recent head 8e33583. Consider uploading reports for the commit 8e33583 to get more accurate results

@@            Coverage Diff             @@
##             main     #160      +/-   ##
==========================================
- Coverage   79.83%   79.79%   -0.04%     
==========================================
  Files          25       27       +2     
  Lines        1314     1475     +161     
==========================================
+ Hits         1049     1177     +128     
- Misses        261      292      +31     
- Partials        4        6       +2     

Flag	Coverage Δ
unittests	79.79% <63.28%> (-0.04%)	⬇️

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

Impacted Files	Coverage Δ
cmd/ddns.go	0.00% <0.00%> (ø)
cmd/ip.go	0.00% <0.00%> (ø)
internal/detector/base.go	100.00% <ø> (ø)
internal/monitor/healthchecks.go	92.77% <92.77%> (ø)
internal/config/config.go	98.51% <100.00%> (+0.08%)	⬆️
internal/config/env.go	100.00% <100.00%> (ø)
internal/monitor/util.go	100.00% <100.00%> (ø)

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