Bug 30074: Add missing extended_attributes rs in DBIC schema

Bug 30074: Add missing extended_attributes rs in DBIC schema

REST API route /patrons has extended_attributes available (x-koha-embed) but it's actually not working.

GET /api/v1/patrons: unhandled exception (Koha::Exceptions::BadParameter)<<Cannot find Koha::Object class for extended_attributes>> Koha::REST::Plugin::Exceptions::__ANON__ /kohadevbo
x/koha/Koha/REST/Plugin/Exceptions.pm (73)

It's missing the relationship named "extended_attributes".

Test plan:
Create a new patron attribute CODE and a value for a patron

Hit /api/v1/patrons?q={"extended_attributes.type":"CODE"}

Also:
1. Apply the regression tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/patrons.t
=> FAIL: Tests fail, boooo
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass!
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>