Move Secure trading logic to this gem

We have decided to get rid of Secure trading (https://github.com/shiftcommerce/shift_commerce-secure_trading) gem, and move that code to flex-ruby-gem.

We should be having the code, in the same way we have the Paypal currently.

- [ ] Move Secure trading code to Ruby gem
- [ ] See if can refactor code between payment gateways after migration.