rtpg722pay: Add encoding-params to the src caps template

rtpg722pay: Add encoding-params to the src caps template

The G722 payload only accepts G722 audio with channels=1, so it must
specify the encoding-params=1 in its src caps, otherwise it causes issues
with farstream which thinks it supports 2 channels G722 and when
confronted with a remote that has G722/8000/2, it will negotiate it
and error out with a not-negotiated when the caps don't intersect
at runtime.

https://bugzilla.gnome.org/show_bug.cgi?id=789878