`extend_schema_serializer` examples do not work on custom pagination classes with nested properties

[w00kie]

Describe the bug
As identified by @AlexChalk in #595 (comment), when using a custom pagination class that has nested properties, the example generation fails to do its magic and falls back to outputting the single example as-is instead of wrapping it with a realistic paginated object.

To Reproduce
Alex gives a good example get_pagination_schema example for a custom pagination class that would fail:

def get_paginated_response_schema(self, schema):
    return {
        "type": "object",
        "properties": {
            "pagination": {
                "type": "object",
                "properties": {
                    "count": {"type": "integer", "example": 10},
                    "next": {
                        "type": "string",
                        "nullable": True,
                        "format": "uri",
                        "example": None,
                    },
                    "previous": {
                        "type": "string",
                        "nullable": True,
                        "format": "uri",
                        "example": None,
                    },
                    "page_size": {"type": "integer", "example": 10},
                    "num_pages": {"type": "integer", "example": 1},
                    "current_page": {"type": "integer", "example": 1},
                },
            },
            "data": schema,
        },
    }

Expected behavior
Ideally the build_listed_example_value in plumbing.py would recursively look through the paginated schema properties to rebuild a complete example and not fail with a warning when it finds a property without an example.

[w00kie]

Thanks for merging this! 💯