feat(alerting): Add Incident.io alerting provider (#972)

* feat(alerting): added incident.io provider alerting.

* Tests: added incident.io provider unit tests.

* Documentation: added incidentio documentation.

* Refactor: Changed documentation + types to an alphabetical order.

* Refactor: change wrong comment.

Co-authored-by: Maksim Zhylinski <maksim@zhylinski.me>

* Update README.md

Co-authored-by: Maksim Zhylinski <maksim@zhylinski.me>

* Update alerting/provider/incidentio/incident_io.go

Co-authored-by: Julien Limoges <limoges@users.noreply.github.com>

* Update alerting/provider/incidentio/incident_io.go

Co-authored-by: Julien Limoges <limoges@users.noreply.github.com>

* Update alerting/provider/incidentio/incident_io.go

Co-authored-by: Julien Limoges <limoges@users.noreply.github.com>

* Update alerting/provider/incidentio/incident_io.go

Co-authored-by: Julien Limoges <limoges@users.noreply.github.com>

* Refactor: changed alertSourceID to url.

* Refactor: changed documentation.

* Refactor: refactored tests, removed status from config.

* Readme: updated docs.

* Refactor: removed duplication key in favor of ResolveKey.

* Refactor: change variable format.

* Feat + Test: added support for passing metadata and source url, added resolved-with-metadata-source-url test case.

* Refactor: chaned variable naming

* Update alerting/config.go

* Update README.md

* Update README.md

* Update README.md

* Apply suggestions from code review

* Refactor: sort var by abc

---------

Co-authored-by: Maksim Zhylinski <maksim@zhylinski.me>
Co-authored-by: Julien Limoges <limoges@users.noreply.github.com>
Co-authored-by: TwiN <twin@linux.com>

config/config.go

@@ -421,6 +421,7 @@ func validateAlertingConfig(alertingConfig *alerting.Config, endpoints []*endpoi
 		alert.TypeTelegram,
 		alert.TypeTwilio,
 		alert.TypeZulip,
+		alert.TypeIncidentIO,
 	}
 	var validProviders, invalidProviders []alert.Type
 	for _, alertType := range alertTypes {