add options to use char* in arguments

[angrest]

The functions publish, subscribe and unsubscribe take String& as arguments, the implementation then passes those as char* (via c_str()) onwards.
In my sketch, I've removed all String instances and only use char*. Thus it would be very helpful (and straightforward to be implemented) if those three functions would be changed to use char* as arguments. To maintain compatibility, the String-versions could call the char* implementations via the same c_str() transformation.