Callback manager in SignalR C++ client should use ints instead of strings for ids

### Is there an existing issue for this?

- [X] I have searched the existing issues

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

The callback id managed by the callback_manager class tracks info by converting an int into a string. This seems like a less than optimal way to manage resources. That's a string allocation and overhead that could be better managed if the id remained as int.

### Describe the solution you'd like

In order to reduce the impact of string allocations and other string-related slowdowns, is it possible to convert the callback manager to use ints instead of strings?

### Additional context

_No response_