[BUG] Vendor extensions in request body are ignored

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you search for related issues/PRs?
• What's the version of OpenAPI Generator used?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

Vendor extensions added to the request body section of the specification are not available in mustache templates.

openapi-generator version

3.4.1

OpenAPI declaration file content or url

Command line used for generation

paths:
  /users:
    post:
      summary: Adds a new user
      requestBody:
        x-foo: "extension" # <- not available in mustache templates
        content:
          application/json:
            schema:      # Request body contents
              type: object
              properties:
                id:
                  type: integer
                name:
                  type: string
              example:   # Sample object
                id: 10
                name: Jessica Smith
      responses:
        '200':
          description: OK

Steps to reproduce

Generate server API from the snippet above, using a custom bodyParts.mustache template containing {{{vendorExtensions.x-foo}}}, which will be empty.

Related issues/PRs

Suggest a fix

DefaultCodegen.fromRequestBody should probably copy the vendor extensions from RequestBody to CodegenParameter

Start with DefaultCodegen.fromRequestBody and trace how RequestBody becomes a CodegenParameter, then inspect the custom bodyParts.mustache reproduction using vendorExtensions.x-foo. Done means the x-foo extension from the request body is available in the mustache template when generating the server API.