Public setter for HttpClient in Java Client

Please make class com.microsoft.signalr.**HttpClient** and function com.microsoft.signalr.HttpHubConnectionBuilder.**withHttpClient** public.

Right know they are package-private, so the only way for us to set up anything is using setHttpClientBuilderCallback method.
That is not ideal, because we have to remember everything we set up inside our OkHttpClient.
For example, we were re-setting all interceptors, but totally forgot to also copy proxy settings.

Passing our own HttpClient would solve these problems.

[Android]