feat/add tag for primary keys (lean testing)

[jilyat]

Add a tag to '_pk' columns in the droughty_schema.yml file to enable lean testing.

[Terroface]

@jilyat in the meantime, I tend to use this command:

dbt test -s test_name:unique

[LewisCharlesBaker]

@Terroface @jilyat - Wouldn't Amir's solution above suffice? What would be the value add of the tag?