feature: toggle whether to build tests for all columns or just primary keys

[Terroface]

Being able to only generate tests for primary keys would allow large projects to use the dbt build statement without it being prohibitively slow.

This would likely sit in dbt_project.yaml

Being able to test_overwrite should still work in this case, as you might want to ensure you have specific tests for crucial non-pk columns even when you are using the newly envisaged pk-only test setting.