Fix not processing enums in cpp-pistache-server (#8886)

* Fix not processing enums in cpp-pistache-server

Defining a reusable enum as a component schema results in an empty
class. Following changes are made:

- activate 'postProcessModelsEnum' in 'AbstractCppCodegen'
- modify model templates for the 'cpp-pistache-server' project

As 'postProcessModelsEnum' is now available in the 'AbstactCppCodegen'
the 'enumVars' variables are now available in mustache templates for all
cpp based code generators.

As the 'AbstractCppCodegen' was touched all cpp based
samples were updated.

* fixes encountered on real world

* PR fixes

Co-authored-by: mfyuce <mfyuce@netas.com.tr>

samples/server/petstore/cpp-pistache/model/Order.cpp

@@ -59,6 +59,7 @@ void to_json(nlohmann::json& j, const Order& o)
         j["status"] = o.m_Status;
     if(o.completeIsSet())
         j["complete"] = o.m_Complete;
+    
 }
 
 void from_json(const nlohmann::json& j, Order& o)
@@ -93,6 +94,7 @@ void from_json(const nlohmann::json& j, Order& o)
         j.at("complete").get_to(o.m_Complete);
         o.m_CompleteIsSet = true;
     } 
+    
 }
 
 int64_t Order::getId() const
@@ -198,6 +200,7 @@ void Order::unsetComplete()
     m_CompleteIsSet = false;
 }
 
+
 }
 }
 }