Implement endpoint POST /api/v2/manage/customers to create a customer.

• return 201 in case of success
• should allow to set a field
• check returned object is uniform with other endpoints
• if a required field is missing, return 400
• if a field value has the wrong type, return 400
• return 403 if the user is denied permission to write customer
• create method in the business layer
• deprecate POST /manage/customers/add
• openapi: document endpoint
• openapi: deprecate POST /manage/customers/add