Create the items client methods

[LautaroPetaccio]

To complete this task the following APIs should be queryable from the client:

• GET: /${address}/items
• GET: /items/${id}
• GET: /collections/${collectionId}/items
• PUT: /items/${item.id} and POST: /items/${item.id}/files to save an item and upload their files.
• DELETE: /items/${item.id}

Take into consideration the following:

• The types that currently live in the builder UI must be moved to either @dcl/schemas or the client itself.
• The fromX and toX methods must be included in the client. They don't need to be exported.