feat(alerting): Add Incident.io alerting provider

[ImTheCurse]

Summary

• Added Incident.io as an alerting provider.
• Added testing for Incident.io alerting provider.
• Added documentation.

Fixes #967

Checklist

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

[ImTheCurse]

@TwiN could you review the PR?

[limoges]

Thanks for taking the time to implement the feature.

I left a few comments. They are all with the intent of improving the default UX of a Gatus user. Let me know what you think.

I'll look at this tomorrow and do some manual testing to see if there are issues (e.g. configure locally, run status, let it resolve alerts).

[ImTheCurse]

@TwiN mind rechecking the PR?

[limoges]

Thanks for adding the source-url and metadata, I did a small manual test with a config and was able to validate the metadata/attributes and source-url.

This is what an alert looks like:

LGTM, I think this would deploy without issue for our usecase.

[codecov]

Codecov Report

Attention: Patch coverage is 83.47826% with 19 lines in your changes missing coverage. Please review.

Project coverage is 75.91%. Comparing base (ea2b7c4) to head (6c516f9).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
alerting/provider/incidentio/incident_io.go	83.33%	13 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #972      +/-   ##
==========================================
+ Coverage   75.78%   75.91%   +0.12%     
==========================================
  Files          74       75       +1     
  Lines        6768     6883     +115     
==========================================
+ Hits         5129     5225      +96     
- Misses       1332     1345      +13     
- Partials      307      313       +6     

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[ImTheCurse]

@TwiN Mind rechecking?

[TwiN]

Almost there!

[TwiN]

@ImTheCurse Thank you for the contribution!