pulse: Add pa_operation_set_state_callback() API

pulse: Add pa_operation_set_state_callback() API

[The original commit message didn't have any explanation why this
change is made, so I'll add that information here myself.
--Tanu Kaskinen]

This change is from the developers of a Haskell binding[1]. According
to them, this change isn't strictly necessary, but their code gets
significantly cleaner if they can register an operation callback that
is called when the operation is cancelled due to the context getting
disconnected.

[1] https://github.com/favonia/pulse