Add domain support

[kyleconroy]

A domain is "essentially a data type with optional constraints". You can think of them as type aliases. By using a domain, you can easily create custom types. For example, here's how you could create a KSUID type.

CREATE DOMAIN ksuid AS VARCHAR(27) NOT NULL;

CREATE TABLE foo (
 id ksuid PRIMARY KEY,
 created_at TIMESTAMP DEFAULT NOW()
);

CREATE TABLE bar (
 id ksuid PRIMARY KEY,
 created_at TIMESTAMP DEFAULT NOW(),
 foo_id ksuid REFERENCES foo(id),
 name text NOT NULL,
 slug text NOT NULL UNIQUE
);

The advantage to using a domain is that you'd only need one entry in overrides

overrides:
 - db_type: "ksuid"
 go_type: "github.com/segmentio/ksuid.KSUID"

Replaces #418

[stepan-romankov]

This looks to be supported, except schema scoped domain types like
CREATE DOMAIN public.name TEXT NOT NULL;

[hectorj-thetreep]

except schema scoped domain

It seems to work too, you just have to include the scope in the db_type