Bug 26635: Expand authorised values in to_api method

Bug 26635: Expand authorised values in to_api method

This patch adds the posibility to expand authorised values when to_api
method is called.

The classes where authorised values should expand must implememnt the
_fetch_authorised_values method, and must return a hash like the
following

{
  column_name => Koha::AuthorisedValue->unblessed
  ...
}

This patch will be used in bug 8179, so please test there.

Sponsored-by: Virginia Polytechnic Institute and State University
Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>