Add video_pre_call_banner feature

Setting video_pre_call_banner variable on the inbound leg of a call that subsequently calls bridge will cause that file to play before connecting the call.
The variable points to the file path of a file that is desired to be played on both legs.

The variable video_pre_call_banner can also be set on an outbound B-leg in the {} section of an originate string and if it differs from the one on the A-leg then the disctinct video will be played on that outgoing leg.

If video_pre_call_banner is not set on the outbound leg, video_pre_call_banner_bleg can be set on the A-leg denoting a specific file to play on the outgoing-B leg.

If no value for a specific file to play on the B-leg is found, the same file that is played to the A-leg will also play on the B-leg.

<action application="set" data="video_pre_call_banner=/path/to/video.mp4"/>
<action application="bridge" data="

{video_pre_call_banner=/path/to/bleg_video.mp4}

sofia/internal/sip:3500@some.com"/>

OR

<action application="set" data="video_pre_call_banner=/path/to/video.mp4"/>
<action application="set" data="video_pre_call_banner_bleg=/path/to/bleg_video.mp4"/>
<action application="bridge" data="sofia/internal/sip:3500@some.com"/>