[MLIR] Add getter for the action handler of MLIR context (#197230)
Current implementation of MLIRContext's action handling, requires the
user to hold the stored memory for any observer or breakpoint manager
added to the execution context. Using a getter for the registered action
handler, permits the user to store the observers and breakpoints into
MLIRContext and modify their state later, by retrieving the action
handler and invoking the functor's target. Mainly for attaching new
observers later in the compilation pipeline, after the execution context
got registered.