Add skip_transaction support to single migrations

[pauldps]

This adds support for calling skip_transaction true inside the migrate method in a single migration so that the statements are run outside the default transaction.

Motivation / real example (CockroachDB does not allow that ALTER TABLE inside a transaction):

class ChangeUsernameInUsers::V20240411185756 < Avram::Migrator::Migration::V1
  def migrate
    skip_transaction true

    # Sized strings require experimental flag
    # https://www.cockroachlabs.com/docs/stable/collate#details
    execute "SET enable_experimental_alter_column_type_general = true;"

    # Case-insensitive collation (CockroachDB does not support CITEXT)
    execute "ALTER TABLE users ALTER username TYPE STRING(30) COLLATE \"en-US-u-ks-level2\";"
  end

  def rollback
    # no-op since it alters existing column in backwards-compatible way
  end
end

▸ Error message unimplemented: ALTER COLUMN TYPE is not supported inside a transaction. Query ALTER TABLE users ALTER COLUMN username TYPE STRING(30) COLLATE "en-US-u-ks-level2";.

TODO:

• tests - my local setup isn't ideal (I don't run Docker) so I'm not able to run tests locally atm
• cleanup