Add feature to allow responding incoming INVITE/re-INVITE asynchronously and set the SDP answer

[ming]

Sometimes, it is desirable to delay answering incoming INVITE/re-INVITE, when: A. application needs to perform certain setup which may take some time B. the preparation affects the SDP answer. An example of this is if the application uses third party media, thus it needs to setup the third party media and put the media info in the SDP.

This ticket will add the following:

• Callback on_call_rx_reinvite() in pjsua, and onCallRxReinvite() in pjsua2, for app to receive notification of incoming re-INVITE and decide if it wants to answer asynchronously at a later timing.
• API pjsua_call_answer_with_sdp() for pjsua. And add sdp parameter in CallOpParam for pjsua2, to be used in Call::answer().

[ming]

In 5828:

Fixed #2128: Add feature to allow responding incoming INVITE/re-INVITE asynchronously and set the SDP answer