[PHP] - Add FormDataProcessor to handle nested ModelInterface data

[jtreminio]

When a request has nested models, and one or more of these models has binary data property (file upload), the PHP generators fail to correctly create a valid formdata array.

This is because only the top-level model/data is checked for binary data properties.

Given:

paths:
  '/pet/{petId}/uploadImageFullFormDataNested':
    post:
      tags:
        - pet
      summary: uploads an image attached to a Pet object as formdata
      description: ''
      operationId: uploadImageFullFormDataNested
      parameters:
        - name: petId
          in: path
          description: ID of pet to update
          required: true
          schema:
            type: integer
            format: int64
      responses:
        '200':
          description: successful operation
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ApiResponse'
      security:
        - petstore_auth:
            - 'write:pets'
            - 'read:pets'
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              properties:
                pet:
                  $ref: '#/components/schemas/PetWithFile'

The generated API method signature looks like:

public function uploadImageFullFormDataNested($pet_id, $pet = null, string $contentType = self::contentTypes['uploadImageFullFormDataNested'][0])

Normally all properties in a formdata object would be listed individually in the signature, but in this case it comes via $pet. The existing method of creating formData array will not work because it expects any file upload data to be in the root-level (ie, listed in the method signature), and not nested within an object.

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[jtreminio]

Pinging maintainers:

@jebentier (2017/07), @dkarlovi (2017/07), @mandrean (2017/08), @jfastnacht (2017/09), @ybelenko (2018/07), @renepardon (2018/12)