Allow to specify a type for references in migrations

[Envek]

This allows to specify a type for created foreign key column in references and add_reference in migrations, very handy:

def change
  enable_extension 'uuid-ossp'
  create_table :cars, id: :uuid do |t|
    t.integer :seats
    # And other car-specific things
  end
  create_table :wheels do |t|
    t.references :car, type: :uuid, index: true
    t.integer :radius
    # And other wheel-specific things
  end
end

Also I've changed PostgreSQL test to use this shiny functionality (looks better, eh?). And if anyone will ever remove this (not by reverting this PR), test will fail, showing that folks that use Postgres really need it ;-)

Bonus: documentation for references method.

P.S> After I wrote this PR I've found PR #13959 (which does the same, but little bit lengthy). Also see it for previous discussion. Choose either you like, I'll be happy with any of these PRs merged.

Will close #13959 if merged.

[senny]

can you add Example: above this line? See other entries for the general style.

[senny]

Can you add the following to the commit message:

[Łukasz Sarnacki & Novikov Andrey]

This will credit both authors for the patch.

[senny]

Any specific reason for this test?

[Envek]

No, just followed other tests in this file…

[senny]

@Envek added a couple minor comments. This patch looks good. Let me know when you updated the code.

[Envek]

Updated code, squashed, and rebased on top of current master.

[senny]

@Envek modified the docs and the CHANGELOG slightly.

@lukesarnacki , @Envek thank you for your work 💛