[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.

Start with org.openapitools.codegen.CodegenParamter and compare it with io.swagger.v3.oas.models.parameters.Parameter, then inspect AbstractCppCodegen.java around line 335. Trace how operation-parameter values reach the Qt mustache template and whether the value is scoped to that parameter rather than global additionalProperties. Done means Qt templates can retrieve allowReserved as {{allowReserved}} for parameter encoding.