add PK_GENERATE() and PK_REFERENCE() for SERIAL of Postgres

[mei-rune]

primary key 'id' is SERIAL

CREATE TABLE some_table(
  id SERIAL PRIMARY KEY NOT NULL,
  string_field VARCHAR(50) NOT NULL,
  boolean_field BOOL NOT NULL,
  created_at TIMESTAMP WITH TIME ZONE,
  updated_at TIMESTAMP WITH TIME ZONE
);

CREATE TABLE other_table(
  id SERIAL PRIMARY KEY NOT NULL,
  int_field INT NOT NULL,
  boolean_field BOOL NOT NULL,
  created_at TIMESTAMP WITH TIME ZONE,
  updated_at TIMESTAMP WITH TIME ZONE
);

CREATE TABLE join_table(
  some_id INT NOT NULL,
  other_id INT NOT NULL,
  PRIMARY KEY(some_id, other_id)
);

CREATE TABLE string_key_table(
  id VARCHAR(50) PRIMARY KEY NOT NULL,
  created_at TIMESTAMP WITH TIME ZONE,
  updated_at TIMESTAMP WITH TIME ZONE
);

and data is:

---
- table: 'some_table'
  pk:
    id: PK_GENERATE(key1)
  fields:
    string_field: 'foobar'
    boolean_field: true
    created_at: 'ON_INSERT_NOW()'
    updated_at: 'ON_UPDATE_NOW()'
- table: 'other_table'
  pk:
    id: PK_GENERATE(key2)
  fields:
    int_field: 123
    boolean_field: false
    created_at: 'ON_INSERT_NOW()'
    updated_at: 'ON_UPDATE_NOW()'
- table: 'join_table'
  pk:
    some_id: PK_REFERENCE(key1)
    other_id: PK_REFERENCE(key2)
- table: 'string_key_table'
  pk:
    id: 'new_id'
  fields:
    created_at: 'ON_INSERT_NOW()'
    updated_at: 'ON_UPDATE_NOW()'

[RichardKnop]

@runner-mei Thanks for PR. Will take a look.