Bug 16213: Allow to select hold's itemtype when using API

Bug 16213: Allow to select hold's itemtype when using API

Test plan:
1/ Use your usual "REST testing" tool to place a title-level hold with
an itemtype. The request should look like this:

POST /api/v1/holds
{
    "borrowernumber": 1234,
    "biblionumber": 456,
    "branchcode": "CPL",
    "itemtype": "A"
}

2/ Check that the hold was placed and the itemtype is correctly selected
3/ prove t/db_dependent/api/v1/holds.t

Signed-off-by: Benjamin Rokseth <benjamin.rokseth@kul.oslo.kommune.no>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>