global: introduce pid_type for item_pid

[ntarocco]

• to uniquely identify a record from its PID, Invenio requires the tuple   PID type and value, not only the value. Changed the item_pid field to   be an object type and value.
• changed methods that were expecting item_pid as a string to now expect   an object instead
• removed unused view circulation/items to retrieve the item from the   loan
• added loan parameter to $ref builder functions to allow more flexibility when implementing the method
• added transaction location and user validations on REST loaders
• added schema loader to validate REST request when replacing the item attached to the loan
• code formatted using Black