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

---------

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

alerting/config.go

@@ -18,6 +18,7 @@ import (
 	"github.com/TwiN/gatus/v5/alerting/provider/ntfy"
 	"github.com/TwiN/gatus/v5/alerting/provider/opsgenie"
 	"github.com/TwiN/gatus/v5/alerting/provider/pagerduty"
+	"github.com/TwiN/gatus/v5/alerting/provider/pushover"
 	"github.com/TwiN/gatus/v5/alerting/provider/slack"
 	"github.com/TwiN/gatus/v5/alerting/provider/teams"
 	"github.com/TwiN/gatus/v5/alerting/provider/telegram"
@@ -59,6 +60,9 @@ type Config struct {
 	// PagerDuty is the configuration for the pagerduty alerting provider
 	PagerDuty *pagerduty.AlertProvider `yaml:"pagerduty,omitempty"`
 
+	// Pushover is the configuration for the pushover alerting provider
+	Pushover *pushover.AlertProvider `yaml:"pushover,omitempty"`
+
 	// Slack is the configuration for the slack alerting provider
 	Slack *slack.AlertProvider `yaml:"slack,omitempty"`