Feature request: implement a subscriber API to align with other SDK offerings

[MikelAlejoBR]

Is your feature request related to a problem? Please describe.

We are trying to log toggle status changes so that we know when exactly those changes have been applied to the application, but we have no way to subscribe to those events.

Describe the solution you'd like
A solution similar to the JDK's SDK which allows us to create subscribers to different events so that our applications can act upon those events.

Describe alternatives you've considered
For now we will probably manually keep track of the status of the toggles and wrap the is_enabled call so that when we notice a status change we can print a log. Either that or try to override the fetch_and_load_features function somehow.

Additional context
N/A