Filtering on mediatype should filter first on mediatype, then encoding

[jdolitsky]

Instead of passing an array (slice) of acceptable mediatype strings, we should instead pass an array of objects such as

{
  mediaType: "application/vnd.unknown.config.v1", 
  supportedEncodings: [
    "+json",
    "+yaml"
  ]
}

[shizhMSFT]

@jdolitsky We might need drop the "allow media type" by #130

[shizhMSFT]

Closing as --allow-media-type is dropped.