add videosource endpoints from Asterisk 14.2

[chadxz]

The bridges resource now exposes two new operations:

• POST /bridges/{bridgeId}/videoSource/{channelId}: Set a video source in a
  multi-party mixing bridge
• DELETE /bridges/{bridgeId}/videoSource: Remove the set video source,
  reverting to talk detection for the video source

https://wiki.asterisk.org/wiki/display/AST/Asterisk+16+Bridges+REST+API#Asterisk16BridgesRESTAPI-setVideoSource
https://wiki.asterisk.org/wiki/display/AST/Asterisk+16+Bridges+REST+API#Asterisk16BridgesRESTAPI-clearVideoSource

[chadxz]

Also add the following fields to Bridge type:

video_mode: string (optional) - The video mode the bridge is using. One of 'none', 'talker', or 'single'.
video_source_id: string (optional) - The ID of the channel that is the source of video in this bridge, if one exists.

[chadxz]

Also add BridgeVideoSourceChanged type

A new event, BridgeVideoSourceChanged, has been added for bridges.
Applications subscribed to a bridge will receive this event when the source
of video changes in a mixing bridge.