Bug 7317: (followup) Migrate endpoint to OpenAPI

Bug 7317: (followup) Migrate endpoint to OpenAPI

This patch moves the current endpoint implementation from Swagger2 to
the OpenAPI plugin.

It also takes advantage of the overloaded Koha::Illrequest::TO_JSON method
which has now the option to embed what's needed for the REST api.

The path spec is adjusted to fit OpenAPI, and some minor fixes are
applied:
- Missing 'metadata' query param
- 'ill' permissions should be required instead of 'borrowers'
- Full test coverage

To test:
- Apply  this patch
- Run:
  $ kshell
 k$ prove t/db_dependent/api/v1/illrequests.t
=> SUCCESS: Tests pass!
- Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Magnus Enger <magnus@libriotech.no>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Benjamin Rokseth <benjamin.rokseth@kul.oslo.kommune.no>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>