Changes to Utwórz nowe powiązanie produktu/wariantu z ofertą zewnętrzną

the response property product_id became nullable for the status 201

added the new optional request property account_id

added the new optional request property product_variant_id

added the new optional request property variant_id

the request property product_id became optional

added the optional property external_account_id to the response with the 201 status

added the optional property variant_id to the response with the 201 status