Facets should not execute for ?shape=array|object

[simonw]

Split off from #255 - there's no point executing the facet SQL for the ?_shape=array and ?_shape=object API responses.

[simonw]

The underlying mechanics for the _extras mechanism described in #262 may help with this.

[simonw]

Fixing this would unblock this issue for switching datasette-graphql to using datasette.client internally: simonw/datasette-graphql#61

[simonw]

This is a lot easier to implement now that we have a ?_nofacet=1 option from #1350.