net/webrtc: multi producer support in webrtcsrc

- Add a new structure Session
  - manage each producer using a session
  - avoid send EOS when a session terminates, instead keep running
    waiting for any new producer to connect

- Maintain a bin element per session
  - each session bin encapsulates webrtcbin and the decoder if needed
    as well as the parser and filter if requested by the application
    (through request-encoded-filter)
  - this will be helpful to cleanup the session's respective elements
    when the corresponding producer terminates the session

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1339>

docs/plugins/gst_plugins_cache.json

@@ -9307,12 +9307,24 @@
                 ],
                 "klass": "Source/Network/WebRTC",
                 "pad-templates": {
+                    "audio_%%s_%%u": {
+                        "caps": "audio/x-raw(ANY):\napplication/x-rtp:\naudio/x-opus:\n",
+                        "direction": "src",
+                        "presence": "sometimes",
+                        "type": "GstWebRTCSrcPad"
+                    },
                     "audio_%%u": {
                         "caps": "audio/x-raw(ANY):\napplication/x-rtp:\naudio/x-opus:\n",
                         "direction": "src",
                         "presence": "sometimes",
                         "type": "GstWebRTCSrcPad"
                     },
+                    "video_%%s_%%u": {
+                        "caps": "video/x-raw(ANY):\napplication/x-rtp:\nvideo/x-vp8:\nvideo/x-h264:\nvideo/x-vp9:\nvideo/x-h265:\nvideo/x-av1:\n",
+                        "direction": "src",
+                        "presence": "sometimes",
+                        "type": "GstWebRTCSrcPad"
+                    },
                     "video_%%u": {
                         "caps": "video/x-raw(ANY):\napplication/x-rtp:\nvideo/x-vp8:\nvideo/x-h264:\nvideo/x-vp9:\nvideo/x-h265:\nvideo/x-av1:\n",
                         "direction": "src",
@@ -9374,12 +9386,24 @@
                 "klass": "Source/Network/WebRTC",
                 "long-name": "WebRTCSrc",
                 "pad-templates": {
+                    "audio_%%s_%%u": {
+                        "caps": "audio/x-raw(ANY):\napplication/x-rtp:\naudio/x-opus:\n",
+                        "direction": "src",
+                        "presence": "sometimes",
+                        "type": "GstWebRTCSrcPad"
+                    },
                     "audio_%%u": {
                         "caps": "audio/x-raw(ANY):\napplication/x-rtp:\naudio/x-opus:\n",
                         "direction": "src",
                         "presence": "sometimes",
                         "type": "GstWebRTCSrcPad"
                     },
+                    "video_%%s_%%u": {
+                        "caps": "video/x-raw(ANY):\napplication/x-rtp:\nvideo/x-vp8:\nvideo/x-h264:\nvideo/x-vp9:\nvideo/x-h265:\nvideo/x-av1:\n",
+                        "direction": "src",
+                        "presence": "sometimes",
+                        "type": "GstWebRTCSrcPad"
+                    },
                     "video_%%u": {
                         "caps": "video/x-raw(ANY):\napplication/x-rtp:\nvideo/x-vp8:\nvideo/x-h264:\nvideo/x-vp9:\nvideo/x-h265:\nvideo/x-av1:\n",
                         "direction": "src",
@@ -9439,12 +9463,24 @@
                 ],
                 "klass": "Source/Network/WebRTC",
                 "pad-templates": {
+                    "audio_%%s_%%u": {
+                        "caps": "audio/x-raw(ANY):\napplication/x-rtp:\naudio/x-opus:\n",
+                        "direction": "src",
+                        "presence": "sometimes",
+                        "type": "GstWebRTCSrcPad"
+                    },
                     "audio_%%u": {
                         "caps": "audio/x-raw(ANY):\napplication/x-rtp:\naudio/x-opus:\n",
                         "direction": "src",
                         "presence": "sometimes",
                         "type": "GstWebRTCSrcPad"
                     },
+                    "video_%%s_%%u": {
+                        "caps": "video/x-raw(ANY):\napplication/x-rtp:\nvideo/x-vp8:\nvideo/x-h264:\nvideo/x-vp9:\nvideo/x-h265:\nvideo/x-av1:\n",
+                        "direction": "src",
+                        "presence": "sometimes",
+                        "type": "GstWebRTCSrcPad"
+                    },
                     "video_%%u": {
                         "caps": "video/x-raw(ANY):\napplication/x-rtp:\nvideo/x-vp8:\nvideo/x-h264:\nvideo/x-vp9:\nvideo/x-h265:\nvideo/x-av1:\n",
                         "direction": "src",