Add metadata generation support

[willosborne]

Adds CLI support to back up changes to spec from #183 .
If metadata is defined it will recursively attempt to instantiate the objects set within. It will resolve refs too, but only on top-level objects.

i.e. you can have (approx, missed some detail out)

"metadata": [
   {
      "$ref": "xyz"
   }
]

but not

"metadata": [
   {
     property: {
       "$ref": "xyz"
     }
   }
]

Note that this also removes support for arbitrary additional top level properties, since this is what metadata replaces.

[linux-foundation-easycla]

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• ✅ login: willosborne / name: Will Osborne (8d98914, 601d43a)