Add support for identifying whether a Session is currently busy or not

[WSL-Ben]

Is your feature request related to a problem?

I am trying to create a list of Sessions that can be re-used as long-running connections, and would like to know if there is a method that can be used to determine whether a specific Session is currently busy or not.

An example of something similar to what I want to do:

cpr::Session get_session() {
    for(auto& session : _sessions) {
      if(session->isBusy()){
        continue;
      }
      return session;
    }
}

Possible Solution

I have been unable to find any workaround or solution.

Alternatives

No response

Additional Context

No response