[Bug] User object unexpectedly updates UID when logging in

### [REQUIRED] Please fill in the following fields:

* Pre-built SDK from the [website](https://firebase.google.com/download/cpp) or open-source from this repo: open-source (self built)
* Firebase C++ SDK version: 11.0.1
* Main Firebase Components in concern: Auth
* Other Firebase Components in use: -
* Platform you are using the C++ SDK on: Windows
* Platform you are targeting: Windows

### [REQUIRED] Please describe the question here:

We noticed that a `User` object obtained from `current_user()` dynamically updates its UID and other user properties to match the current user’s properties when logging in or out.

This prevents keeping a reference to a previously logged in user, e.g. to delete a previously logged in anonymous user after logging in with user credentials.

This behavior does not match the iOS or Android SDK‘s behavior, where an object obtained from the equivalent `current_user()` method will not change when logging or out, and as far as we could tell it is also not documented.

I‘d appreciate feedback to know whether this is intended behavior or a bug, and whether there is a way to work around this behavior. I'm probably just missing something here.