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.

test/spec/fixtures/data.sql

@@ -812,3 +812,15 @@ INSERT INTO designers(id, name) VALUES (1, 'Sid Meier'), (2, 'Hironobu Sakaguchi
 
 TRUNCATE TABLE videogames CASCADE;
 INSERT INTO videogames(id, name, designer_id) VALUES (1, 'Civilization I', 1), (2, 'Civilization II', 1), (3, 'Final Fantasy I', 2), (4, 'Final Fantasy II', 2);
+
+TRUNCATE TABLE students CASCADE;
+INSERT INTO students(id, code, name) VALUES (1, '0001', 'John Doe'), (2, '0002', 'Jane Doe');
+
+TRUNCATE TABLE students_info CASCADE;
+INSERT INTO students_info(id, code, address) VALUES (1, '0001', 'Street 1'), (2, '0002', 'Street 2');
+
+TRUNCATE TABLE country CASCADE;
+INSERT INTO country(id, name) VALUES (1, 'Afghanistan'), (2, 'Algeria');
+
+TRUNCATE TABLE capital CASCADE;
+INSERT INTO capital(id, name, country_id) VALUES (1, 'Kabul', 1), (2, 'Algiers', 2);