Create the items client methods

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.