Being able to assign keys for oxp functions

_Spec kept up-to-date with the below comments_
The assignment of functions to keys would be done by the player.
The oxp should be able to declare a "key-assignable" function, similar to addFrameCallback(function).
The oxp should be able to redeclare it.
Should the context be taken into account? (PlayerEntityControls.status?pause?gui while inflight?)
