sendrecv: Implement remote-offerer option for JS example

sendrecv: Implement remote-offerer option for JS example

Now you can check the "Remote offerer" checkbox in the JS example to
force the peer to send the SDP offer. This involved implementing
support for receiving the OFFER_REQUEST message in the C example.

As a side-effect of this, the C example will no longer send
OFFER_REQUEST automatically when the --our-id option is passed. It
will only do so when the --remote-offerer option is explicitly passed.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-examples/-/merge_requests/31>