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.