Better validation of all config file fields (#6090)

Add Validate() to serverConfig to call it at server
startup and in Admin SetConfig handler to minimize
errors scenario after server restart.

pkg/event/target/webhook.go

@@ -21,6 +21,7 @@ import (
 	"crypto/tls"
 	"crypto/x509"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"net"
 	"net/http"
@@ -38,6 +39,17 @@ type WebhookArgs struct {
 	RootCAs  *x509.CertPool `json:"-"`
 }
 
+// Validate WebhookArgs fields
+func (w WebhookArgs) Validate() error {
+	if !w.Enable {
+		return nil
+	}
+	if w.Endpoint.IsEmpty() {
+		return errors.New("endpoint empty")
+	}
+	return nil
+}
+
 // WebhookTarget - Webhook target.
 type WebhookTarget struct {
 	id         event.TargetID