[BUG][php] body name from x-codegen-request-body-name is ignored

Bug Report Checklist

• [x ] Have you provided a full/minimal spec to reproduce the issue?
• [x ] Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• [x ] Have you searched for related issues/PRs?
• [x ] What's the actual output vs expected output?
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Description

After a conversion from swagger 2 to OAS3, i see a new parameter in my yaml file in my operation:
x-codegen-request-body-name: ListOfDefinitions

This parameter is ignored during the php compilation and I got an object named InlineObject instead

openapi-generator version

5.1.0

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: test
  description: test
  version: 1.0.0
servers:
  - url: /
paths:
  /definitions:
    put:
      summary: test
      operationId: putTest
      requestBody:
        description: test
        content:
          application/json:
            schema:
              type: object
              properties:
                label:
                  type: string
                  description: label
        required: true
      responses:
        204:
          description: successful operation
          content: {}
      x-codegen-request-body-name: ListOfDefinitions

Generation Details

Steps to reproduce

Related issues/PRs

Suggest a fix

Start with the supplied OpenAPI 3.0.1 declaration and the PHP generation for PUT /definitions, then trace how x-codegen-request-body-name is handled. Done means the generated PHP uses ListOfDefinitions instead of InlineObject, with coverage for this declaration and output.