Support ActionMailbox callbacks for Definition requests

[andyw8]

Similar to what we do for ActiveRecord callbacks, we could support the 3 callbacks that ActionMailbox provides:

• around_processing
• before_processing
• after_processing

https://api.rubyonrails.org/classes/ActionMailbox/Callbacks.html

[aniketpatidar]

I would like to tackle this one!

[domingo2000]

@andyw8 I just added a PR to solve this by reusing the implementation for ActiveRecord callbacks. Happy to receive feedback!