Implement GET /voucher-storage/voucher-number/start

Overview:
The voucher-number sequence start value must be configurable by the tenant. In order to facilitate this we need to provide a way to get the current start value of this sequence, e.g. so we can display it in the UI invoice/voucher settings area.

In order to get the current start value of the voucher-number sequence we need an API which returns that information.

E.g.
SELECT * FROM <sequence name>;

or better yet:
SELECT start_value FROM <sequence name>;

The sequenceNumber schema can be reused to return the value.

Acceptance Criteria:

• the start value of the voucher-number sequence is returned
• Unit tests are updated