bridge_softmix / app_confbridge: Add support for REMB combining.

This change adds the ability for multiple REMB reports in
bridge_softmix to be combined according to a configured
behavior into a single report. This single report is sent
back to the sender of video, which adjusts the encoding bitrate
to be at or below the bitrate of the report. The available
behaviors are: lowest, highest, and average. Lowest uses the
lowest received bitrate. Highest uses the highest received
bitrate. Average goes through the received bitrates adding
them to the previous average and creates a new average.

Other behaviors can be added in the future and the existing
average one may be adjusted, but this provides the foundation
to do so.

Support for configuring which behavior to use has been
added to app_confbridge.

ASTERISK-27804

Change-Id: I9eafe4e7c1f72d67074a8d6acb26bfcf19322b66

configs/samples/confbridge.conf.sample

@@ -239,6 +239,10 @@ type=bridge
                            ; A REMB frame contains receiver estimated maximum bitrate information. By creating a combined
                            ; frame and sending it to the sources of video the sender can be influenced on what bitrate
                            ; they choose allowing a better experience for the receivers. This defaults to 0, or disabled.
+;remb_behavior=average     ; How the combined REMB report for an SFU video bridge is constructed. If set to "average" then
+                           ; the estimated maximum bitrate of each receiver is used to construct an average bitrate. If
+                           ; set to "lowest" the lowest maximum bitrate is forwarded to the sender. If set to "highest"
+                           ; the highest maximum bitrate is forwarded to the sender. This defaults to "average".
 
 ; All sounds in the conference are customizable using the bridge profile options below.
 ; Simply state the option followed by the filename or full path of the filename after