Support multibyte language for export_openapi_schema

[lion-man44]

I'd like to use export_openai_schema command in Japanese way.
But the command can't export it.

Because of this.

django-ninja/ninja/management/commands/export_openapi_schema.py

Lines 79 to 84 in 39f40b3

	result = json.dumps(
	schema,
	cls=NinjaJSONEncoder,
	indent=options["indent"],
	sort_keys=options["sort_keys"],
	)

        result = json.dumps(
            schema,
            cls=NinjaJSONEncoder,
            indent=options["indent"],
            sort_keys=options["sort_keys"],
        )

# sample code
>>> data
{'id': 1, 'name': 'ほげ☺️'}
>>> json.dumps(data)
'{"id": 1, "name": "\\u307b\\u3052\\u263a\\ufe0f"}'
>>> json.dumps(data, ensure_ascii=False)
'{"id": 1, "name": "ほげ☺️"}'

I think it's much better if ninja can support multibyte language, since ninja is a Japanese word :)
Thanks.