Support `djangorestframework-dataclasses` `Union` type

[johnthagen]

This PR added support for Union types in djangorestframework-dataclasses

• https://github.com/oxan/djangorestframework-dataclasses/pull/82
• https://github.com/oxan/djangorestframework-dataclasses#unionfield

I wasn't sure if drf-spectacular would need to be adjusted to support this or not? Just wanted to report it here for awareness as this is an exciting new feature.

It was released in 1.3.0

• https://github.com/oxan/djangorestframework-dataclasses/releases/tag/v1.3.0

[awwithro]

This would be very handy, I'm currently trying to get this to work myself. As is, it currently throws a warning could not resolve serializer field "UnionField...