Improve callback class set API in RpcBindingParam

[khotyn]

SOFARPC provide a method in RpcBindingParam for users to set callback class when users use callback invocation type, but the method accept a String as type:

public void setCallbackClass(String callbackClass) {
}

We should improve this method to a more strong typed method, such as

public void setCallbackClass(Class<? extends SofaResponseCallback> callbackClass) {
}

so users are less likely to make mistakes.

[leizhiyuan]

这个有一些历史原因，不方便改，新增的话已经有不少了，对于想使用类或者对象的，可以使用

setCallbackHandler方法，直接传入对象。