AV policy data and file name extensions

[tomcrane]

The RFC for AV policy settings says

...then Engine looks up "video-mp4-720p" and sees, in some form, "Transcode with "System preset: 'Mp4 HLS 720p'; Extension: 'mp4'", so it calls Elastic Transcoder with that information.

and later

While the names of these policies can technically be anything, the following convention will be used to simplify understanding:

<channel>-<format>-<quality>

https://github.com/dlcs/protagonist/blob/a96a3a94d64a575671965e07e3012796f5ecd349/docs/rfcs/015-iiif-av-delivery-channel-settings.md

However, we are using that convention as formal syntax - we're learning what file extension to save the output with.

This should be more explicit, so the DLCS does not have to parse <channel>-<format>-<quality>.

Suggestion:

    "DeliveryChannelMappings": {
      "video-mp4-480p":  { "data": "System preset: Generic 480p 16:9",     "extension": "mp4"  },
      "video-webm-720p": { "data": "System preset: Webm 720p(webm)",       "extension": "webm" },
      "audio-mp3-128k":  { "data": "System preset: Audio MP3 - 128k(mp3)", "extension": "mp3"  }
    }