Add forward compatible return types (#620)

Add bool return type to `isValidEvent()` of
`PerfDataConsumerForm` and `VCenterShipMetricsForm`
to ensure forward compatibility with ipl-html and ipl-stdlib.

Author: BastianLedererIcinga

library/Vspheredb/Web/Form/PerfdataConsumerForm.php

@@ -56,7 +56,7 @@ public function assemble()
         $this->addButtons(isset($implementation), 'implementation');
     }
 
-    public function isValidEvent($event)
+    public function isValidEvent($event): bool
     {
         if ($event === self::ON_DELETE) {
             return true;

library/Vspheredb/Web/Form/VCenterShipMetricsForm.php

@@ -121,7 +121,7 @@ protected function addConsumerConfig($consumer)
         }
     }
 
-    public function isValidEvent($event)
+    public function isValidEvent($event): bool
     {
         if ($event === self::ON_DELETE) {
             return true;