Correctly dump `:bigserial` primary keys on PG

Opted for polymorphism on the connection adapter, rather than the type
objects, as putting it there would require adding methods to all types
that ultimately only affects a small number of types and only on
PostgreSQL. I've got some ideas for separating schema concerns into
another object, this will be refactored onto those once I flesh that
out.

Fixes #15968.
Could you please add a `when :string` case here to cover https://github.com/rails/rails/issues/14169#issuecomment-47828168.