Problem/Motivation
See parent meta for motivations on introducing this component.
At the moment, in order to resolve callables with various DX features (such as service notation), this is either being implemented manually or using the controller_resolver service. The problem is this is a strange coupling, we shouldn't need a service that is primarily designed to deal with Request objects to resolve callbacks for completely unrelated subsystems.
Proposed resolution
A new service called "callback_resolver" that standardises the approach for resolving callbacks, decoupled from controllers.
Down the track: use the callback_resolver ControllerResolver in places where a callable is required from a "definition".
Remaining tasks
Validate and patch.
