Implement GET /voucher-storage/voucher-number

Overview:
We need to generate sequential voucher numbers.

Pertains to this field in the acquisitions interface fields document:

Approach

• Create a sequence in the tenantAPI (when the module is enabled for a tenant) - start at 0 (can be changed later on via another API)
• Return the next value in the sequence when the GET /voucher-storage/voucher-number endpoint is called.

See the API Listing document for details

NOTE: contrary to earlier discussions, the voucher-number is not required to be unique.

Acceptance Criteria:

• RAML and module descriptor are updated
• The sequence is created by the tenantAPI
• The API is implemented as described above
• Unit tests are updated