Bug 33170: Refactor MARCItemFieldsToOrder to make adding more fields trivial

Bug 33170: Refactor MARCItemFieldsToOrder to make adding more fields trivial

There is no need for this code to have a hard coded list of fields directly in the code.
Any invalid keys would be skipped anyway.
If we refactor this code then adding new fields will be much simpler.

Test Plan:
1) Set up your MARCItemFieldsToOrder, verify everything is working
2) Apply this patch
3) Restart all the things!
4) Verify there has been no change to the MARCItemFieldsToOrder
   functionality

Signed-off-by: Andrew Fuerste-Henry <andrewfh@dubcolib.org>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>