Alter table add constraint postgres

[griffio]

Minimal support for Postgres alter table add constraint as learning how all this works.

  ALTER TABLE test
  ADD CONSTRAINT idx_external_event_id
  UNIQUE (external_event_id);

The changes are made here on sqldelight as the postgressql grammer was moved from sql-psi

Noticed that issue was raised in AlecKazakova/sql-psi#286

Have signed contributors agreement.

---

TODOs

Check constraints - there needs to be support for type expr functions like char_length

e.g

ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5);

[AlecKazakova]

awesome! Thank you!