Add no-decode option

[mickel8]

Not decoding and rendering incoming streams might be useful when benchmarking WHEP servers. In such cases, we might not be interested in opening a video player window.

This PR adds a new option that allows for discarding incoming streams.

Alternatively, we could split it into two options - no_decode and no_render.

[mickel8]

Looks like these changes break ICE connection 👀

After some time, I am getting:

[WHEP] ICE completed
0:00:14.004695723 30194 0x7dab680d7260 INFO               webrtcbin gstwebrtcbin.c:7725:on_rtpbin_sender_timeout:<webrtcbin0> session 0 ssrc 2209846735 sender timeout
0:00:14.004754317 30194 0x7dab680d7260 INFO               webrtcbin gstwebrtcbin.c:7725:on_rtpbin_sender_timeout:<webrtcbin0> session 0 ssrc 29349805 sender timeout
0:00:28.209583252 30194 0x7dab680d7260 INFO               webrtcbin gstwebrtcbin.c:7803:on_rtpbin_timeout:<webrtcbin0> session 0 ssrc 2209846735 timeout
0:00:28.209631460 30194 0x7dab680d7260 INFO               webrtcbin gstwebrtcbin.c:7803:on_rtpbin_timeout:<webrtcbin0> session 0 ssrc 29349805 timeout
0:00:30.570000349 30194 0x5d6e29032230 INFO        GST_ELEMENT_PADS gstelement.c:1016:gst_element_get_static_pad: found pad dtlssrtpdec0:sink
0:00:30.570079957 30194 0x5d6e290437b0 INFO               webrtcbin gstwebrtcbin.c:1660:_update_ice_connection_state_task:<webrtcbin0> ICE connection state change from completed(3) to failed(4)
[WHEP] [ERR] [src/whep-client.c:whep_ice_connection_state:634] ICE failed
[WHEP] Disconnecting from server (ICE failed)

[lminiero]

I think that's because the pipeline is broken. IMO the solution is not to skip creating the decodebin and its pads, but probably put something like a fakesink or something like that when you don' want to do anything with the media. The pipeline must be complete.