Support DTMF via SIP INFO

[nanang]

Although there seem to be no standard that describes the feature specification clearly, it seems to be widely used. Having it implemented in the library should be practical, especially for PJSUA2 apps (note that there is already an implementation in pjsua sample app using PJSUA API). Thanks to David Wichter for the suggestion.

Considering that this is a proprietary feature (see ​here) and it may not be desirable on some system, e.g: SIP INFO message burst may introduce congestion on the network and may cause load spike on SIP server, the feature should be configurable and by default it is disabled.

This ticket will introduce pjsua_dtmf_method to specify the DTMF sending method.

PJSUA API
To send DTMF as SIP INFO:
Set pjsua_call_send_dtmf_param.method to PJSUA_DTMF_METHOD_SIP_INFO when calling pjsua_call_send_dtmf().

To get the notification of incoming DTMF:
Use on_dtmf_digit2() callback.

PJSUA2 API
To send DTMF as SIP INFO:
Set CallSendDtmfParam::method to PJSUA_DTMF_METHOD_SIP_INFO when calling Call::sendDtmf().

To get the notification of incoming DTMF:
Use Call::onDtmfDigit() callback.

[riza]

In 5834:

Close #2036: Support DTMF via SIP INFO.