Add tablespace support to Index class

[Markus Holtermann]

For now, the django.db.models.indexes.Index class determines a non-default tablespace from its model field if the index is defined on a single field (Field.db_tablespace) or from a model's _meta.db_tablespace for multi-column indexes. Indexes on expressions, however, won't be able to identify which columns they refer to.

The initial API design would be to add db_tablespace or tablespace as an attribute to the Index class and use that if provided, and fallback to the current behavior if not given.

[GitHub <noreply@…>]

In 3297dede:

Fixed #28046 -- Added the db_tablespace parameter to class-based indexes.

Thanks Markus Holtermann and Tim Graham for reviews.