feat: add one to one relationship for resource embedding

[steve-chavez]

Closes #1984. One to one relationships are detected when:

• There's a foreign key which is also a primary key:

-- one-to-one rel between students and students_info
CREATE TABLE test.students(
  id int
, code text
, name text
, primary key(id, code)
);

CREATE TABLE test.students_info(
  id int
, code text
, address text
, primary key(id, code)
, foreign key (id, code) references test.students(id, code)
);

• There's a foreign key with an unique constraint

-- one-to-one rel between country and capital
CREATE TABLE test.country(
  id int primary key
, name text
);

CREATE TABLE test.capital(
  id int primary key
, name text
, country_id int unique
, foreign key (country_id) references test.country(id)
);

(There's a blog post by Vertabelo that makes this clearer)

This works across the board(tables, views, rpc, mutations, computed rels can override them). O2O rels are a refinement of M2O rels, so no news rels are added and there is no potential for new disambiguation conflicts.

[laurenceisla]

Just to comment that this query runs on pg 15 beta 3 and builds correctly. Did not check the code, though.