Bug 37253: Enhance POST /checkouts endpoint to accept barcode or item_id

Bug 37253: Enhance POST /checkouts endpoint to accept barcode or item_id

This patch adds external_id as a body param in POST /checkouts which acts as the items barcode.
This enhances the checkouts route to checkout items directly via barcode which could be useful for external
tools like Koha Offline Circulation Tool

Test plan:

a) apply patch
b) enable system preference RESTBasicAuth
c) check out an item via an API testing tool. Be sure that item is not checked out already.

   Auth: username: koha  &  password: koha
   Body JSON:
   {
     "external_id" : "39999000011418",
     "library_id": "CPL",
     "patron_id": 5
   }
   POST http://localhost:8081/api/v1/checkouts
d) check response code 201 and response content

Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>