Add a 'chainId' attribute to VideoConversion contribution

Currently VideoConversion can only specify a converter trough its converter attribute:

<extension point="videoConversions" target="org.nuxeo.ecm.platform.video.service.VideoService">

    <videoConversion converter="convertToMP4" height="480" name="MP4 480p" rendition="true"/>
    <videoConversion converter="convertToWebM" height="480" name="WebM 480p" rendition="true"/>
    <videoConversion converter="convertToOgg" height="480" name="Ogg 480p" rendition="true"/>

</extension>

This could be restraining when the converter needs to execute a different command depending e.g. on the mime type of the video file to convert. E.g. Addtional parameters need to be provided to the command for AVI video file in order to be properly converted.

A chainId attribute would allow such flexibility as an *automation chain or scripting can provide logical expressions to choose different converters to execute. PictureConversion contribution provides such a parameter.