SyncMBeanImpl.syncExternalUsers doesn't check for foreign IDP

in the light of OAK-4264 i found that SyncMBeanImpl.syncExternalUsers doesn't verify that the given external-ids belong to the same IDP that the bean has been created for.

my expectation for the result-messages was an operation key for (FOREIGN) but instead the user might even be synchronized depending on the behavior of the IDP or result in nsi if the a given userID is (by coincidence) not known to the IDP.