Add functionality to remove an identity and / or clear their traits

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

When using the Flagsmith SDK in an application and identifying users, customers may provide the option for their users to opt out of tracking. As Flagsmith could be considered by some as a form of tracking (depending on what data is being sent to the platform) it would be useful to have a way to remove an identity and / or their traits from the Flagsmith platform.

**Describe the solution you'd like**

Add functionality to fully remove an identity and/or their traits from the Flagsmith platform.

**Describe alternatives you've considered**

Currently the platform allows you to send `null` trait values to remove traits from a user. This is sufficient for a number of use cases but it requires the developers to know which traits may have been set.

**Additional context**

N/A

**Considerations**

Is this an endpoint that we would want to expose to client API keys or should this be restricted to server API keys only? I would say the latter but perhaps this could be configurable.

gz#479