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

include/asterisk/bridge.h

@@ -903,6 +903,15 @@ int ast_bridge_is_video_src(struct ast_bridge *bridge, struct ast_channel *chan)
  */
 void ast_bridge_remove_video_src(struct ast_bridge *bridge, struct ast_channel *chan);
 
+/*!
+ * \brief Converts an enum representation of a bridge video mode to string
+ *
+ * \param video_mode The video mode
+ *
+ * \retval A string representation of \c video_mode
+ */
+const char *ast_bridge_video_mode_to_string(enum ast_bridge_video_mode_type video_mode);
+
 enum ast_transfer_result {
 	/*! The transfer completed successfully */
 	AST_BRIDGE_TRANSFER_SUCCESS,

include/asterisk/stasis_bridges.h

@@ -58,6 +58,10 @@ struct ast_bridge_snapshot {
 	unsigned int num_channels;
 	/*! Number of active channels in the bridge. */
 	unsigned int num_active;
+	/*! The video mode of the bridge */
+	enum ast_bridge_video_mode_type video_mode;
+	/*! Unique ID of the channel providing video, if one exists */
+	AST_STRING_FIELD_EXTENDED(video_source_id);
 };
 
 /*!