Add support for With clause

[Ryan Heard]

Multiple database providers including ​PostgreSQL, ​MySQL and ​Oracle provide a WITH clause which can eliminate duplicate operations by putting results into a variable which can be reused.

To add this to the Django ORM, I propose adding a with_clause method to QuerySet. Note that with is a defined keyword and cannot be used.

To imagine how this might work, we can look at the example found in the PostgreSQL documentation and try to rebuild it using the Django ORM

Orders.objects.with_clause(
    regional_sales=Orders.objects.values('region').distinct().annotate(total_sales=Sum('amount')),
    top_regions=F('regional_sales').values('region').filter(total_sales__gt=Sum('total_sales')),
).filter(
    region__in=F('top_regions'),
).values(
    'region', 'product',
).distinct().annotate(
    product_units=Sum('quantity'),
    product_sales=Sum('amount'),
)

[Simon Charette]

Duplicate of #28919.