payment: fix state machine flow when overriding the operation result

When overriding the operation result, for example when transitioning away
from a PENDING state, the fake result from the plugin had PaymentPluginStatus.PROCESSED
hardcoded. This didn't work as the plugin status drives the transition
into the new state.

Testing done: verified we can transition from PENDING to SUCCESS and from PENDING to
PAYMENT_FAILURE using the Adyen plugin.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
jaxrs: fix missing Guice wiring for PaymentGatewayResource

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