Bug 32981: Add endpoint to get AV for given category

Bug 32981: Add endpoint to get AV for given category

For, at least, the ERM module we would like to retrieve the authorised
values for a given category to build a dropdown list with the different options.

It has been decided on bug 17390 to use
GET /authorised_value_categories/:authorised_value_category_id/values

Test plan:
curl -v -s -u koha:koha --request GET  http://kohadev-intra.mydnsname.org:8081/api/v1/authorised_value_categories/LOC/values
Should display the list of LOC

curl -v -s -u koha:koha --request GET  http://kohadev-intra.mydnsname.org:8081/api/v1/authorised_value_categories/xLOCx/values
Should return a 404

Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>