Support for required additionalProperties in schema

When specifying additionalProperties fields as required there's no indication of these.

Example schema:

openapi: 3.0.2
info:
  title: Test
  version: "0.0.1"
paths:
  /example:
    get:
      summary: Example operation
      responses:
        '200':
          description: ok
          content:
            application/json:
              schema:
                allOf:
                  - type: object
                    description: dynamic fields, expect to see `prop1` and `prop2` as required field names here
                    additionalProperties:
                      type: string
                  - required:
                    - prop1
                    - prop2
              example:
                prop1: required prop
                prop2: required prop
                prop3: optional prop

Start by reproducing the OpenAPI schema shown in the issue and inspect how its rendered documentation represents additionalProperties and required fields. Done means required property names such as prop1 and prop2 are visibly distinguished from optional dynamic properties such as prop3; the payload names no files or tests.