Improve avatar field in LSP-3

[frozeman]

Add also game engines.
and platform.

     "avatar": [ // a 3D file avatar mostly in FBX and/or OBJ format, multiple file formats of an avatar can be added
            {
                "hashFunction": ['keccak256(json)','md5(bytes)'],
                "hash": ['string','string'],
                "url": ['string', 'string'],
                "fileType": ['manifest','assetbundle'],
                "engine": 'unity',
                "platform": "unix",
                "version": 1.23.1
            }
        ]