* Support specifying payment method through apps
closes#1525
This is great if you want to offer items with an incentive to use a specific payment method without messing with the rates.
For example, you can have `Item A` which costs 25$ and your store is configured for USDT and BTC. You can create two items, with different prices, where Item A costs 20$ if you pay with bitcoin or 25$ if you paid in dirty fiat pegs
* make code cleaner
* Add Test
* fix test
* fix test
5033cb3186