feat(alerting): Add new providers for Datadog, IFTTT, Line, NewRelic, Plivo, RocketChat, SendGrid, Signal, SIGNL4, Splunk, Squadcast, Vonage, Webex and Zapier (#1224)

* feat(alerting): Add new providers for Datadog, IFTTT, Line, NewRelic, Plivo, RocketChat, SendGrid, Signal, SIGNL4, Splunk, Squadcast, Vonage, Webex and Zapier

Relevant: https://github.com/TwiN/gatus/discussions/1223

Fixes #1073
Fixes #1074

* chore: Clean up code

* docs: Fix table formatting

* Update alerting/provider/datadog/datadog.go

* Update alerting/provider/signal/signal.go

* Update alerting/provider/ifttt/ifttt.go

* Update alerting/provider/newrelic/newrelic.go

* Update alerting/provider/squadcast/squadcast.go

* Update alerting/provider/squadcast/squadcast.go

config/config.go

@@ -444,6 +444,7 @@ func validateAlertingConfig(alertingConfig *alerting.Config, endpoints []*endpoi
 	alertTypes := []alert.Type{
 		alert.TypeAWSSES,
 		alert.TypeCustom,
+		alert.TypeDatadog,
 		alert.TypeDiscord,
 		alert.TypeEmail,
 		alert.TypeGitHub,
@@ -452,21 +453,34 @@ func validateAlertingConfig(alertingConfig *alerting.Config, endpoints []*endpoi
 		alert.TypeGoogleChat,
 		alert.TypeGotify,
 		alert.TypeHomeAssistant,
+		alert.TypeIFTTT,
 		alert.TypeIlert,
 		alert.TypeIncidentIO,
 		alert.TypeJetBrainsSpace,
+		alert.TypeLine,
 		alert.TypeMatrix,
 		alert.TypeMattermost,
 		alert.TypeMessagebird,
+		alert.TypeNewRelic,
 		alert.TypeNtfy,
 		alert.TypeOpsgenie,
 		alert.TypePagerDuty,
+		alert.TypePlivo,
 		alert.TypePushover,
+		alert.TypeRocketChat,
+		alert.TypeSendGrid,
+		alert.TypeSignal,
+		alert.TypeSIGNL4,
 		alert.TypeSlack,
+		alert.TypeSplunk,
+		alert.TypeSquadcast,
 		alert.TypeTeams,
 		alert.TypeTeamsWorkflows,
 		alert.TypeTelegram,
 		alert.TypeTwilio,
+		alert.TypeVonage,
+		alert.TypeWebex,
+		alert.TypeZapier,
 		alert.TypeZulip,
 	}
 	var validProviders, invalidProviders []alert.Type