Add API to register custom SDP comparison callback

[sauwming]

Since SDP can be customized (for example, it can contain custom attributes), app may wish to add its own SDP comparison logic on top of the implementation provided by pjmedia_sdp_session_cmp().

The PR will provide API which allows app to register a custom SDP comparison callback.

[nanangizz]

While here, pjmedia_sdp_session_cmp() docs may be better to be updated too on which SDP fields it compares.

Btw, perhaps it is good idea to let pjmedia_sdp_session_cmp() invokes the callback too (also, make the callback to be part of sdp_cmp instead of sdp_neg)?