feat(webhooks): add route to get all available webhook events

pkg/models/webhooks.go

@@ -20,6 +20,7 @@ import (
 	"code.vikunja.io/api/pkg/events"
 	"code.vikunja.io/api/pkg/user"
 	"code.vikunja.io/web"
+	"sort"
 	"sync"
 	"time"
 	"xorm.io/xorm"
@@ -71,6 +72,17 @@ func RegisterEventForWebhook(event WebhookEvent) {
 	})
 }
 
+func GetAvailableWebhookEvents() []string {
+	evts := []string{}
+	for e := range availableWebhookEvents {
+		evts = append(evts, e)
+	}
+
+	sort.Strings(evts)
+
+	return evts
+}
+
 func (w *Webhook) Create(s *xorm.Session, a web.Auth) (err error) {
 	// TODO: check valid webhook events
 	w.CreatedByID = a.GetID()