Missing support trigram indexes / index opclasses

Hi, given the release of [trigram indexes](https://www.cockroachlabs.com/docs/v23.1/trigram-indexes#how-do-trigram-indexes-work), I have tried to implement them in my django app. However, the migrations are failing. From what I can tell, django-cockroachdb hasn't been updated to support them yet (as seen [here](https://github.com/cockroachdb/django-cockroachdb/blob/a73b5b7ad0f758bd8150933f11ba6dacfd145965/django_cockroachdb/schema.py#L46-L52))

The output from `python manage.py sqlmigrate` confirms this:

When using engine `django_cockroachdb`, output shows `CREATE INDEX "study_firstName" ON "study_study" USING gist ("firstName");`

When using engine `django.db.backends.postgresql`, output shows `CREATE INDEX "study_firstName" ON "study_study" USING gist ("firstName" gist_trgm_ops);`