Bug 22456: Add Koha::Hold::CancellationRequest(s) class(es)

Bug 22456: Add Koha::Hold::CancellationRequest(s) class(es)

This patch adds the Koha::Object-derived classes for cancellation
requests.

Tests are added for the overloaded ->store method.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Hold/CancellationRequest.t
=> SUCCESS: Tests pass!
3. Sign off :-D

Sponsored-by: Montgomery County Public Libraries
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>