New PJSUA-LIB account registration state callback to report more callback data (thanks Johan Lantz for the suggestion)

[bennylp]

Some application would like to inspect the response of REGISTER response, and currently this is not possible since the response is not reported in on_reg_state() callback.

This ticket adds a new callback, on_reg_state2(), which also adds the whole response message as well as possibly other infos as parameter.

[nanang]

(In [3322]) Close #1138:

• Added new pjsua registration status callback on_reg_state2(), it includes the whole info from the lower layer registration callback pjsip_regc_cb().