add a future in `RawMaster` waiting for `ExternalEvent` received from any PDU frame

[jimy-byerley]

the PDU header contains a ExternalEvent stuct whose bits are enabled when the slave reports some interruptions on its side and registers::external_event_mask is set.
At the moment these are ignore by RawMaster but it could be useful to have a way to get this info out.
Since a slave interrupt is reported in the first PDU it sends back, it doesn't concern the containing PDU and should be made available separately by RawMaster. An notify or something like this might be suited.