Add GTIN to TradeItem

Pre-conditions:

• Knowing the credentials of an administrator;

• Obtaining the user's access token;

• Obtaining the data of at least two trade items whose manufacturerOfTradeItem parameter is not blank, with the use of the GET /api/tradeItems endpoint.

Description from the ticket:
Add the field "GTIN" to the tradeitem so that we can support the storage of a GTIN. GTIN is currently up to 14 digits long, though a valid number could be less (GTIN-8, 13, ...) and could change. Additionally GTIN's currently do not support non-digit characters. GTIN pamphlet.

Acceptance Criteria

• GTIN is optional and not required.

• A new value object exists in reference data that encapsulates GTIN and it's validation.

• Update JSON schema of TradeItem

• Enforce global uniqueness of the GTIN