Extended OrderItem model and List All Promotion Tiers methods

[bandraszyk]

Make sure that the following methods are provided:

• Server side: Promotions.ListAllTiers(?is_avai=able=true/false); https://docs.voucherify.io/v2018-08-01/reference#list-promotion-tiers
• Client side: Promotions.ListAllTiers(?is_avai=able=true/false); https://docs.voucherify.io/v2018-08-01/reference#list-promotion-tiers-client-side

Make sure that the following methods can work with extended Order Item model:

• Order; https://docs.voucherify.io/v2018-08-01/reference#the-order-object

• Order Items; https://docs.voucherify.io/v2018-08-01/reference#the-order-item-object\

• Create Order; https://docs.voucherify.io/v2018-08-01/reference#create-order

• Update Order; https://docs.voucherify.io/v2018-08-01/reference#update-order

• Validate Voucher; https://docs.voucherify.io/v2018-08-01/reference#validate-voucher

• Validate Voucher (client side); https://docs.voucherify.io/v2018-08-01/reference#vouchers-validate

• Validate Promotions; https://docs.voucherify.io/v2018-08-01/reference#validate-promotions-1

• Redeem Voucher; https://docs.voucherify.io/v2018-08-01/reference#redeem-voucher

• Redeem Voucher (client side); https://docs.voucherify.io/v2018-08-01/reference#redeem-voucher-client-side

• Redeem Promotion; https://docs.voucherify.io/v2018-08-01/reference#redeem-promotion

Close the issue if no operation is needed or provide a solution with next release.