[cpp pistache] fix leading number in model names and apply modelNamePrefix in all cases

Created by: etherealjoy

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

• Allow prefixing API Names and Model Names and their files, this allow usage of conflicting model names from different specs or to prevent a number in a class name and file name.
• escape nameInCamelCase for variables with a leading number.
• default prefix is empty string no change in current behaviour
• other c++ petstore sample code remains unchanged.
• Same behaviour as Qt5 client/server

usage example

java -jar ~/openapi-generator/modules/openapi-generator-cli/target/openapi-generator-cli.jar  \
generate -g cpp-pistache-server -i ~/test/swagger.yaml -o ~/gennew  \
--additional-properties modelNamePrefix=TET

With this sample spec

openapi: 3.0.0
info:
  version: 1.0.0
  title: Check 5gmmCapability
servers:
  - url: 'http://localhost:8080'
  
paths:
  /Check5gmmCapability:
    get:
      summary: Check property beginning with a number
      operationId: list
      tags:
        - Check
      responses:
        '200':
          description: Everythings gonna be alright
          headers:
            x-next:
              description: A link to the next page of responses
              schema:
                type: string
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Content'
components:
  schemas:
    5gmmCapability:
      type: object
      properties:
        item1:
          type: string
        item2:
          type: integer
        5gItem:
          type: integer
    Content:
      type: object
      properties:
        type:
          $ref: '#/components/schemas/5gmmCapability'

we have something like this

class TET5gmmCapability --> File name model\TET5gmmCapability.h class TETCheckApiImpl --> File name impl\TETCheckApiImpl.h class TETCheckApi --> File name api\TETCheckApi.h class TETModelBase --> File name model\TETModelBase.h main --> File name TETCheckApiMainServer.cpp

Fixes #604 (closed)

@stkrwork @MartinDelille @fvarose @ravinikam