feat(alerting): Add Pushover provider (#405)

* Add a new Pushover provider (#129)
- Adds new provider named Pushover with corresponding tests
- Adds Pushover as a provider to the configuration and adjusts test accordingly
- Adds Pushover to alerting_test.go, provider.go and type.go
- Updates the readme with configuration details

* Correct import order

* Fix some missing pushover references

* Apply suggestions from code review

* Rename application-key to application-token for Pushover

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Co-authored-by: TwiN <twin@linux.com>

config/config.go

@@ -352,6 +352,7 @@ func validateAlertingConfig(alertingConfig *alerting.Config, endpoints []*core.E
 		alert.TypeNtfy,
 		alert.TypeOpsgenie,
 		alert.TypePagerDuty,
+		alert.TypePushover,
 		alert.TypeSlack,
 		alert.TypeTeams,
 		alert.TypeTelegram,