res_pjsip_session Added rtcp stats result vector into the session

Currently, the Asterisk's pjsip_session module does not keeping the
rtcp's stats info after it was removed. But by adding the results
vector and keeping it until session is destroying, it can give more
useful information for other modules.

ASTERISK-28253

Change-Id: Ib25c2d3fc4da084aecfde2a82c1b1d733bd64fa5