Add option to set RX telephone-event PT

[sauwming]

To implement the proposal by @nanangizz as described in #3661:
Allow app overriding si->rx_event_pt via PJSUA callback on_stream_precreate() (and PJSUA2). This can be useful for cases such as:

• if remote doesn't specify telephone-event in its SDP answer
• if remote sends tel-event PT different than the one negotiated in the SDP

To avoid multiple PTs offered, it is recommended to disable PJMEDIA_TELEPHONE_EVENT_ALL_CLOCKRATES. Then application can simply override param->stream_info.info.aud.rx_event_pt to PJMEDIA_RTP_PT_TELEPHONE_EVENTS or the PT expected to be sent by remote.

To fix #3700 as well.

Tested here and it seems to work as expected:
Before patch (if remote sends DTMF PT different than the SDP):

14:00:33.055        strm0x10c9b1928  Bad RTP pt 122 (expecting 96)
14:00:33.075        strm0x10c9b1928  Bad RTP pt 122 (expecting 96)
14:00:33.094        strm0x10c9b1928  Bad RTP pt 122 (expecting 96)
14:00:33.114        strm0x10c9b1928  Bad RTP pt 122 (expecting 96)

After patch:

14:04:55.036            pjsua_app.c  .Incoming DTMF on call 0: 1, using RFC2833 method
14:04:55.276            pjsua_app.c  .Incoming DTMF on call 0: 2, using RFC2833 method
14:04:55.517            pjsua_app.c  .Incoming DTMF on call 0: 3, using RFC2833 method