Improve extensibility of schema migration for custom backends for new Index/Check/Unique and other constraints

[Pavel Tyslacki]

New in django 2.2 Index condition https://code.djangoproject.com/ticket/29547 and Check/Unique constraints https://code.djangoproject.com/ticket/29641 are a very useful, but have a some issues with extensibility by custom backends:

• sql generation for migrations splitted to schema and indexes/constraints modules, but custom backend for indexes/constraints modules don't appliable
• schema logic can be improved for extensibility to avoid huge overriding as _alter_field method

to fix it proposed:

• move sql generation from Index/CheckConstraint/UniqueConstraint to schema methods (similar to _create_index_sql)
• provide methods for sql generation for index/unique/primary key/foreign key/check constraints

[Tim Graham <timograham@…>]

In 0123b67:

Fixed #30060 -- Moved SQL generation for indexes and constraints to SchemaEditor.

[GitHub <noreply@…>]

In 16c966ff:

Refs #30060, Refs #34217 -- Made SchemaEditor not generate SQL for CheckConstraint if not supported.

The new logic mirrors the logic in SchemaEditor._delete_check_sql()
added in 68ef274bc505cd44f305c03cbf84cf08826200a8.

Thanks Tim Graham for the report.