Allow specifying custom operator classes for PostgreSQL indexes

[vinay karanam]

This is an extension of #28077 to support specifying operator class for indexes.

Usage:

Index(fields=['jsonfield'], index_name='jsonb_idx', operator_class='jsonb_path_ops')

[Tim Graham]

Is this only supported on PostgreSQL? Of course, documentation would also be needed.

[vinay karanam]

AFAICT, this is only supported on psql. I am unable to find similar feature on other databases.

I've updated the corresponding documentation.

[Sergey Fedoseev]

​PR

[Tim Graham]

Is there a need for a separate ticket from #28077? (Is there some additional work needed to close that or could we just retitle it?)

I'm unsure if adding database-specific kwargs like operator_class to Index.__init__() is a good approach, but I don't have an alternative in mind.

[vinay karanam]

#28077 requires almost same amount of work. It is only asking operator support for GinIndex, whereas this adds support for any postgres index. We can close it in favour of this or retitle #28077 and close this ticket.

Regarding code changes, even I was skeptical at first about adding operator_class to Index.__init__() but then I saw Index.create_sql accepts using kwarg which is only valid for postgres index.
If necessary we can have a subclass PGIndex and move postgres specific options into this.

[Tim Graham]

Okay, I added the details from this ticket to #28077 and retitled it.