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
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