Add support for callback helpers

[emorissettegregoire]

This PR introduces support for before_state, after_state, and around_state callbacks within the FlowManager.

These callbacks can now be defined in the flow DSL and are automatically invoked during state transitions:

• before_state runs before the state block,
• after_state runs after the state block, and
• around_state wraps the state block, enabling advanced behaviors like pre- and post-processing.

Additionally, the flow structure has been updated to support nested states, with callbacks as a top-level key.
The structure now looks like this:

@flows = {
  hello: {
    states: {
      say_hello: { block: ..., options: ... },
      ...
    },
    callbacks: { before_state: [ ], around_state: [ ], after_state: [ ] }
  }
}