CDB3-4136 Add validation on classification schema. items.classification

AR:
There is a possibility to create procedure without classification schema

ER:
Change item.classification schema to required while creating procedure

https://procedure-dev.prozorro.sale/api/procedures/642e7459a9f7fec78597aee4
https://procedure-dev.prozorro.sale/api/procedures/642e7569a9f7fec78597aeec
https://procedure-dev.prozorro.sale/api/procedures/642e765da9f7fec78597aef6
https://procedure-dev.prozorro.sale/api/procedures/642e784da9f7fec78597aefc

"classification": { "scheme": "CAV", "id": "04000000-8" },