Allow to define requestBody and parameters in OpenAPI docstrings

[perdy]

To further customisation in OpenAPI docstring it's desirable to allow a request body definition like the following example:

@app.route("/custom-body/", methods=["POST"])
async def custom_body():
    """
    description: Custom body.
    requestBody:
        description: The body request
        required: true
        content:
            multipart/form-data:
                schema:
                    type: object
                    properties:
                        model:
                            type: string
                            format: binary
                            description: The model file
                        name:
                            type: string
                            description: The model name
                        description:
                            type: string
                            description: The model description
    responses:
      200:
        description: OK.
    """
    return {}

Also, a parameters redefinition like:

@app.route("/custom-params/", methods=["GET"])
async def custom_params(name: str):
    """
    description: Custom params.
    parameters:
      - in: query
        name: name
        schema:
            type: string
        required: true
        description: The query param
    responses:
      200:
        description: OK.
    """
    return {}