pjsip / res_rtp_asterisk: Add support for sending REMB

This change allows chan_pjsip to be given an AST_FRAME_RTCP
containing REMB feedback and pass it to res_rtp_asterisk.
Once res_rtp_asterisk receives the frame a REMB RTCP feedback
packet is constructed with the appropriate contents and sent
to the remote endpoint.

ASTERISK-27776

Change-Id: Ic53f821c1560d8924907ad82c4d9c0bc322b38cd