'none' channel makes some fields optional

[donaldgray]

An asset with none delivery-channel is not ingested by Engine - it is effectively a placeholder. To make creation of none assets easier (e.g. for "stub" assets), assets with none delivery-channel should have slightly different validation logic. "origin" and "mediaType" are optional and given a placeholder if omitted.

• "origin" is optional, if none delivery-channel. Default to: "https://example.org/origin"
• An "origin" of "https://example.org/origin" is invalid for any other delivery-channels
• "mediaType" is optional, if none delivery-channel. Default to: "example/example" (uses valid example/ type)
• An "mediaType" of "example/example" is invalid for any other delivery-channels
• Legacy mode should continue to require both "origin" and "mediaType" as they can't supply delivery-channel.