Define required fields in the schemas

Overview:
Make the following fields required in the schemas. There may be additional fields which will need to be made required later on, but many of those are only required when certain conditions are met, so we have to enforce those in code as opposed to in the schemas.

Purchase order:

• vendor (already done - MODORDERS-208)
• orderType

PO Line:

• purchaseOrderId
• acquisitionMethod
• cost
  • currency
• orderFormat (already done - https://github.com/folio-org/acq-models/pull/110)
• title

Acceptance Criteria:

• the fields above are required in their corresponding schemas
• any example, sample, mock data is updated if needed.
• Unit tests are updated as needed
• API tests are updated as needed