Implement event handler system for message passing and callbacks

[bvdmitri]

Summary

• Introduces a generic event-driven callback system (Event{E}, handle_event, invoke_callback) that lets users hook into every major step of the message passing algorithm: rule calls, message products, form constraint application, and marginal
  computation
• Adds MessageProductContext to replace bare product strategy arguments, bundling prod_constraint, form_constraint, check strategy, and callbacks into a single context object
• Refactors variable types with labels, docstrings, and cleaner file organization (variable.jl → generic.jl)

Callback events:

• BeforeMessageRuleCallEvent / AfterMessageRuleCallEvent
• BeforeProductOfTwoMessagesEvent / AfterProductOfTwoMessagesEvent
• BeforeFormConstraintAppliedEvent / AfterFormConstraintAppliedEvent
• BeforeMarginalComputationEvent / AfterMarginalComputationEvent

Callback handlers can be nothing (no-op), a callable, a Tuple of callables, or a Dict keyed by event name. Includes CallbackResultReducer for aggregating results across events.

Other changes:

• Removed hardcoded DefaultMessageProdFn / DefaultMarginalProdFn in favor of MessageProductContext
• Added MethodError hint for mismatched handle_event signatures
• Added label field to RandomVariable, ConstVariable, DataVariable
• Improved docstrings for form constraints and variable types
• Documentation page for callbacks

Test plan

• New test/callbacks_tests.jl covering all event types, handler variants (nothing, function, tuple, dict), reducers, and merging
• Extended test/message_tests.jl for MessageProductContext and callback integration in product computations
• Extended test/variables/random_tests.jl for labels and callback-aware activation
• Existing tests updated for new API (clusters_tests, dependencies_tests, mixture_tests)