Bug 33573: Add public endpoint for cancelling holds

Bug 33573: Add public endpoint for cancelling holds

This patch introduces a public endpoint for cancelling holds.
Cancellation requests are generated when the hold is waiting and
configuration allows requesting cancellation, as the OPAC does right
now.

Tests cover all the use cases.

To test:
1. Apply this patches
2. Run:
   $ ktd --shell
  k$ prove t/db_dependent/api/v1/patrons_holds.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Signed-off-by: Sam Lau <samalau@gmail.com>
Signed-off-by: Emily Lamancusa <emily.lamancusa@montgomerycountymd.gov>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>