helpers: add :skip_gw option

Allow calls to skip the gateway entirely. This can be used for testing
or to migrate payment methods that are already in the gateway.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
generators: update signature in PaymentMethod model for consistency

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
spec: cleanup payment_plugin_spec

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>