Bug 29107: Rename item_type => item_type_id

Bug 29107: Rename item_type => item_type_id

This simple change renames the attribute in the item object API
representation.

To test:
1. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/items.t
=> SUCCESS: Tests pass
2. Run:
   $ git grep 'api/v1/items'
=> SUCCESS: The items routes are not (yet) used in Koha
3. Run:
   $ cd koha-tmpl
   $ git grep --name-only '"embed"'
=> SUCCESS: Only two templates are using embed.
4. Check:
   - register.tt uses it to embed the manager, good
   - parcel.tt uses it to embed items. Alert.
=> SUCCESS: parcel.tt only uses a count on items. No item_type involved.
5. Sign off

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>