Bug 35287: Preparation: Extend AdditionalFields Mixin

Bug 35287: Preparation: Extend AdditionalFields Mixin

The mixin now implements the extended_attributes method to be utilized by any Koha class that implements the AdditionalFields Mixin and is exposed through the REST API
It also implements a strings_map to be utilized in the same fashion as described above. This is useful because additional fields may be an authorised value

Signed-off-by: Edith Speller <edith.speller@ukhsa.gov.uk>
Signed-off-by: Michaela Sieber <michaela.sieber@kit.edu>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Paul Derscheid <paul.derscheid@lmscloud.de>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>