[C++][Pistache] Serialize integer enums if possible (#16080)

In OpenAPI it is possible to define an enum schema containing integers
only.

Similar to the following JSON snippet:
```
...
  "components": {
    "schemas": {
      "size": {
        "type": "integer",
        "description": "Container size",
        "enum": [
          10000,
          20000,
          100000,
          200000,
          300000,
          1000000,
          1200000,
          2500000,
          5000000,
          10000000
        ]
      }
    }
  }
...
```

To correctly serialize this we need to convert to JSON integers. We can
achieve this by feeding nlohmann JSON objects directly with integers
instead of strings.

For the C++ pistache server adapt the enum models to serialize integer
values if possible.

modules/openapi-generator/src/main/resources/cpp-pistache-server/model-source.mustache

@@ -101,11 +101,11 @@ void to_json(nlohmann::json& j, const {{classname}}& o)
         {{#enumVars}}
         {{#-first}}
         case {{classname}}::e{{classname}}::INVALID_VALUE_OPENAPI_GENERATED:
-            j = "INVALID_VALUE_OPENAPI_GENERATED";
+            j = {{#isInteger}}0{{/isInteger}}{{^isInteger}}"INVALID_VALUE_OPENAPI_GENERATED"{{/isInteger}};
             break;
         {{/-first}}
         case {{classname}}::e{{classname}}::{{name}}:
-            j = "{{value}}";
+            j = {{#isInteger}}{{value}}{{/isInteger}}{{^isInteger}}"{{value}}"{{/isInteger}};
             break;
         {{/enumVars}}
     }{{/allowableValues}}{{/isEnum}}{{#vendorExtensions.x-is-string-enum-container}}{{#anyOf}}{{#-first}}to_json(j, o.m_value);{{/-first}}{{/anyOf}}{{/vendorExtensions.x-is-string-enum-container}}
@@ -121,9 +121,9 @@ void from_json(const nlohmann::json& j, {{classname}}& o)
     } {{/required}}
     {{/vars}}
     {{#isEnum}}{{#allowableValues}}
-    auto s = j.get<std::string>();
+    auto s = j.get<{{#isInteger}}{{dataType}}{{/isInteger}}{{^isInteger}}std::string{{/isInteger}}>();
     {{#enumVars}}
-    {{#-first}}if{{/-first}}{{^-first}}else if{{/-first}} (s == "{{value}}") {
+    {{#-first}}if{{/-first}}{{^-first}}else if{{/-first}} (s == {{#isInteger}}{{value}}{{/isInteger}}{{^isInteger}}"{{value}}"{{/isInteger}}) {
      o.setValue({{classname}}::e{{classname}}::{{name}});
     } {{#-last}} else {
      std::stringstream ss;

modules/openapi-generator/src/main/resources/cpp-pistache-server/model-struct-source.mustache

@@ -96,12 +96,12 @@ void to_json(nlohmann::json& j, const {{classname}}& o)
     {{#enumVars}}
     {{#-first}}
     case {{classname}}::e{{classname}}::INVALID_VALUE_OPENAPI_GENERATED:
-         j = "INVALID_VALUE_OPENAPI_GENERATED";
-         break;
+        j = {{#isInteger}}0{{/isInteger}}{{^isInteger}}"INVALID_VALUE_OPENAPI_GENERATED"{{/isInteger}};
+        break;
     {{/-first}}
     case {{classname}}::e{{classname}}::{{name}}:
-         j = "{{value}}";
-         break;
+        j = {{#isInteger}}{{value}}{{/isInteger}}{{^isInteger}}"{{value}}"{{/isInteger}};
+        break;
     {{/enumVars}}
     }{{/allowableValues}}{{/isEnum}}{{#vendorExtensions.x-is-string-enum-container}}{{#anyOf}}{{#-first}}to_json(j, o.m_value);{{/-first}}{{/anyOf}}{{/vendorExtensions.x-is-string-enum-container}}
 }
@@ -117,10 +117,10 @@ void from_json(const nlohmann::json& j, {{classname}}& o)
     }{{/required}}
     {{/vars}}
     {{#isEnum}}{{#allowableValues}}
-    auto s = j.get<std::string>();
+    auto s = j.get<{{#isInteger}}{{dataType}}{{/isInteger}}{{^isInteger}}std::string{{/isInteger}}>();
     {{#enumVars}}
     {{#-first}}
-    if{{/-first}}{{^-first}}else if{{/-first}}(s == "{{value}}") {
+    if{{/-first}}{{^-first}}else if{{/-first}}(s == {{#isInteger}}{{value}}{{/isInteger}}{{^isInteger}}"{{value}}"{{/isInteger}}) {
         o.value = {{classname}}::e{{classname}}::{{name}};
     } {{#-last}} else {
         std::stringstream ss;