refactor: PKcols in table, ViewKeyDependency type

* Get PKcols inside tables - done with SQL for tables and
  with an additional step in Haskell for views.

  This fixes an fk column being considered as a pk column on views
  and corrects the test added on
  https://github.com/PostgREST/postgrest/pull/1875/files/1d549768580310e18aac4ffa6dbd01c5b77934a7#r853674126

* classify view key dependencies in SQL

* remove Column from Relationship

* Merge cols/fcols in Relationship and
  ensure allM2ORels and allViewsKeyDependencies fk columns
  are ordered - done by attnum in SQL

* Cardinality now contains relColumns instead of Relationship -
  this simplifies getJoinConditions.

src/PostgREST/DbStructure/Table.hs

@@ -27,6 +27,7 @@ data Table = Table
   , tableInsertable  :: Bool
   , tableUpdatable   :: Bool
   , tableDeletable   :: Bool
+  , tablePKCols      :: [FieldName]
   }
   deriving (Show, Ord, Generic, JSON.ToJSON)