core / pjsip: Add support for grouping streams together.

In WebRTC streams (or media tracks in their world) can be grouped
together using the mslabel. This informs the browser that each
should be synchronized with each other.

This change extends the stream API so this information can
be stored with streams. The PJSIP support has been extended
to use the mslabel to determine grouped streams and store
this association on the streams. Finally when creating the
SDP the group information is used to cause each media stream
to use the same mslabel.

ASTERISK-27379

Change-Id: Id6299aa031efe46254edbdc7973c534d54d641ad