Core/SmartScript: create SMART_ACTION_SET_CORPSE_DELAY

by Vincent-Michael, updates #16488

New smart action SMART_ACTION_SET_CORPSE_DELAY:
- Allows setting the delay of corpse decay time in SmartScript

(PR opened on request from Rushor)
Constructive feedback is welcome.

src/server/game/AI/SmartScripts/SmartScriptMgr.cpp

@@ -1214,6 +1214,7 @@ bool SmartAIMgr::IsEventValid(SmartScriptHolder& e)
         case SMART_ACTION_REMOVE_GO_FLAG:
         case SMART_ACTION_SUMMON_CREATURE_GROUP:
         case SMART_ACTION_RISE_UP:
+        case SMART_ACTION_SET_CORPSE_DELAY:
             break;
         default:
             TC_LOG_ERROR("sql.sql", "SmartAIMgr: Not handled action_type(%u), event_type(%u), Entry %d SourceType %u Event %u, skipped.", e.GetActionType(), e.GetEventType(), e.entryOrGuid, e.GetScriptType(), e.event_id);