res/ari/resource_bridges: Add the ability to manipulate the video source

In multi-party bridges, Asterisk currently supports two video modes:
 * Follow the talker, in which the speaker with the most energy is shown
   to all participants but the speaker, and the speaker sees the
   previous video source
 * Explicitly set video sources, in which all participants see a locked
   video source

Prior to this patch, ARI had no ability to manipulate the video source.
This isn't important for two-party bridges, in which Asterisk merely
relays the video between the participants. However, in a multi-party
bridge, it can be advantageous to allow an external application to
manipulate the video source.

This patch provides two new routes to accomplish this:
(1) setVideoSource: POST /bridges/{bridgeId}/videoSource/{channelId}
    Sets a video source to an explicit channel
(2) clearVideoSource: DELETE /bridges/{bridgeId}/videoSource
    Removes any explicit video source, and sets the video mode to talk
    detection

ASTERISK-26595 #close

Change-Id: I98e455d5bffc08ea5e8d6b84ccaf063c714e6621

rest-api/api-docs/events.json

@@ -161,6 +161,7 @@
 				"BridgeMerged",
 				"BridgeBlindTransfer",
 				"BridgeAttendedTransfer",
+				"BridgeVideoSourceChanged",
 				"ChannelCreated",
 				"ChannelDestroyed",
 				"ChannelEnteredBridge",
@@ -358,6 +359,20 @@
 				}
 			}
 		},
+		"BridgeVideoSourceChanged": {
+			"id": "BridgeVideoSourceChanged",
+			"description": "Notification that the source of video in a bridge has changed.",
+			"properties": {
+				"bridge": {
+					"required": true,
+					"type": "Bridge"
+				},
+				"old_video_source_id": {
+					"required": false,
+					"type": "string"
+				}
+			}
+		},
 		"BridgeBlindTransfer": {
 			"id": "BridgeBlindTransfer",
 			"description": "Notification that a blind transfer has occurred.",