feat: add one to one relationship for embedding

BREAKING CHANGE: For the cases where one to one relationships are
detected, json objects will be returned instead of json arrays of length
1.

If you wish to override this behavior, you can use computed
relationships to return arrays again.

src/PostgREST/DbStructure/Relationship.hs

@@ -37,12 +37,13 @@ data Relationship = Relationship
 
 -- | The relationship cardinality
 -- | https://en.wikipedia.org/wiki/Cardinality_(data_modeling)
--- TODO: missing one-to-one
 data Cardinality
   = O2M {relCons :: FKConstraint, relColumns :: [(FieldName, FieldName)]}
   -- ^ one-to-many
   | M2O {relCons :: FKConstraint, relColumns :: [(FieldName, FieldName)]}
   -- ^ many-to-one
+  | O2O {relCons :: FKConstraint, relColumns :: [(FieldName, FieldName)]}
+  -- ^ one-to-one, this is a refinement over M2O so operating on it is pretty much the same as M2O
   | M2M Junction
   -- ^ many-to-many
   deriving (Eq, Ord, Generic, JSON.ToJSON)