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

**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