Update the Media Gallery Item schema to allow for AudioObject, ImageObject, & VideoObject @types

[gravenp]

The "Media Gallery Item" Ceramic schema needs to be updated to account for audio, image, and video being added to the gallery.

The @type property should allow 3DModel, AudioObject, ImageObject, & VideoObject values.

• Update the spec schema
• Update the schema used by the Cadastre on Ceramic mainnet (i.e., https://tiles.ceramic.community/document/k3y52l7qbv1frypnqvn6izc45risq0ay37zajc99d3zbqnb3kxd2958j4rv9b74lc)