Bug 31236: Add ability to send custom item fields via SIP using Template Toolkit

Bug 31236: Add ability to send custom item fields via SIP using Template Toolkit

Koha has been able to send arbitrary item fields via the "item_field" parameter in the config. We have partners that need the ability to created custom item fields from templates, as the item_fields feature cannot accomplish what they need. We need to add a templated custom field feature for items, similar to what we have for patrons.

Test Plan:
1) Apply this patch
2) Choose a SIP login to use, edit that account and add the following
   *inside* the login section:
   <custom_item_field field="IN" template="[% item.itemnumber %]" />
3) Restart SIP
4) Run an item information query
5) Note the itemnumber is sent in the IN field!

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Edit: fixed tests count