Add functionality analogous to `onChange` in JS SDK to all server side clients

[matthewelwell]

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

Unable to act based on changes to features when in local evaluation mode.

Describe the solution you'd like

Ability to attach 'listener' functions to the flagsmith client so that, when the environment is updated, these listeners are called if a given feature has changed.

Describe alternatives you've considered

• With the addition of real time flags, we will know when something has changed, but not a specific feature. The ability to add listeners would be extensible for when a real time update is received.

Additional context

N/a