Bug 28272: (QA follow-up) Add 'manager' to cashups definition

Bug 28272: (QA follow-up) Add 'manager' to cashups definition

The embeddable 'manager' relation field was missing from the
specification file and as such calls using this embed would result in a
500 error since we introduced 'additionalProperties: false'.

Test plan
1/ Enable 'UseCashRegisters'
2/ Navigate to the register details page
3/ Inspect the network traffic and note a 500 error on the cashups api
call
4/ Apply the patch
5/ The 500 should have gone away and been replaced with an empty
resultset 200 page.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>