Generate and negotiate telephone-event with multiple clock-rates in SDP offer/answer

[nanang]

Currently PJMEDIA will only generate and answer SDP with one telephone-event format, i.e: telephone-event/8000. But in the stream session, it will send DTMF using timestamp correctly according to the audio codec clock rate.

This ticket will update the PJMEDIA behavior in SDP offer/answer:

• in generatig SDP offer, it will generate one or more telephone-event formats, one for each clock rate, in accordance with the clock rates of the offered audio codecs.
• in generating SDP answer, it will select telephone-event clock rate based on the clock rate of the selected audio codec.