method getPayloadMeta corrected

New features:

• the method getPayloadMeta in CrestApi corrected,
• the snapshotTime in GlobalTagDto was set as an optional parameter (like insertionTime).

CrestApi/CrestModel.h

@@ -62,7 +62,7 @@ public:
     std::string description;
     std::string release;
     std::optional<std::string> insertionTime;
-    std::string snapshotTime;
+    std::optional<std::string> snapshotTime;
     std::string scenario;
     std::string workflow;
     std::string type;