Bug 36481: Add GET /libraries/:library_id/cash_registers

Bug 36481: Add GET /libraries/:library_id/cash_registers

This patch adds the mentioned route. For the task it:

* Adds Koha::Cash::Register->to_api_mapping with trivial mappings
* Adds a cash_register object definition on the API spec
* Adds a controller to handle requests
* Adds tests for the new endpoint

To test:
1. Apply this patch
2. Run:
   $ ktd --shell
  k$ qa
=> SUCCESS: All green! Tests pass!
3. Play with Postman!
4. Sign off :-D

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>