Allow custom properties in our json schema

[sobolevn]

We introduced jsonschema validation in e7e1a0f, but, it does not work for custom test plugin to pytest-mypy-plugins: https://github.com/typeddjango/djangorestframework-stubs/actions/runs/8105756625/job/22154578423?pr=547

I will fix this in 3.1.1

[sobolevn]

and typeddjango/django-stubs#1986