Add Killing Blow Ability ID to EVENT_PVP_KILL_FEED_DEATH

[Anthonysc]

It is already possible (although incomplete, and somewhat imprecise) to associate the killing blow ability with EVENT_PVP_KILL_FEED_DEATH events allowing a custom killfeed with more situationally useful information. However, this requires registering an additional callback to EVENT_COMBAT_EVENT. Including the abilityID of the Killing Blow ability in the events fired for EVENT_PVP_KILL_FEED_DEATH would simplify the construction of custom killfeeds with more detailed information.

Edit: Might also be nice to get ClassIDs in this but KB abilityID is the important ask.