[REQ][Qt] Add allowReserved variable to mustache for operation parameter

Is your feature request related to a problem? Please describe.

I am working on parameter styling support for the Qt code generator. I came across the problem that the allowReserved variable is missing in the template. I need this variable for the parameter encoding, because they are either percent encoded or not.

Describe the solution you'd like

I would like to be able to retrieve the allowReserved variable in mustache with {{allowReserved}}

Describe alternatives you've considered

There are no alternatives as long is I am aware.

Additional context

I was searching the project and came across the class org.openapitools.codegen.CodegenParamter that is missing the allowReserved variable while the class io.swagger.v3.oas.models.parameters.Parameter contains it.
A solution that might work is to change the AbstractCppCodegen.java on line 335f. and retrieve the variable from the OpenAPI Object and put it in additionalProperties but I am not sure if thats the way to go because I would put it on a global mustache level and not on the parameter of the according operation.

Beginne mit org.openapitools.codegen.CodegenParamter und vergleiche es mit io.swagger.v3.oas.models.parameters.Parameter. Untersuche dann AbstractCppCodegen.java um Zeile 335. Verfolge, wie die Werte von Operationsparametern die Qt mustache template erreichen und ob der Wert auf diesen Parameter beschränkt ist statt auf globale additionalProperties. Erledigt ist die Aufgabe, wenn Qt templates allowReserved als {{allowReserved}} für die Parameter-Codierung abrufen können.