enhancement: add ability to define `encoding` option on operations with complex serialization

[GamerGirlandCo]

From the openapi specification website:

Complex Serialization in Form Data

[...] For more complex scenarios, such as nested arrays or JSON in form data, you need to use the contentType keyword to specify the media type for encoding the value of a complex field. Consider Slack incoming webhooks for an example. A message can be sent directly as JSON, or the JSON data can be sent inside a form field named payload like so (before URL-encoding is applied):

payload={"text":"Swagger is awesome"}

This can be described as:

openapi: 3.0.0
info:
  version: 1.0.0
  title: Slack Incoming Webhook
externalDocs:
  url: https://api.slack.com/incoming-webhooks

servers:
  - url: https://hooks.slack.com

paths:
  /services/T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX:
    post:
      summary: Post a message to Slack
      requestBody:
        content:
          application/json:
            schema:
              $ref: "#/components/schemas/Message"

          application/x-www-form-urlencoded:
            schema:
              type: object
              properties:
                payload: # <--- form field that contains the JSON message
                  $ref: "#/components/schemas/Message"
            encoding:
              payload:
                contentType: application/json

      responses:
        "200":
          description: OK

components:
  schemas:
    Message:
      title: A Slack message
      type: object
      properties:
        text:
          type: string
          description: Message text
      required:
        - text

Perhaps the ContentUnit struct could be extended to include a map field that maps property names to mime-type strings, and an Encoding function could be added to populate said map?