Allow defining a payment method redirecting directly to a specific Tpay channel

[jakubtobiasz]

Description 📄

Currently, the Pay By Link payment method allows us to pick any of the available channels provided by Tpay. However, we need to provide a way to add a payment method that redirects us to a specific channel associated with a given payment method.

So when I define a "traditional" pay by link, I should be able to choose any of the available channels. However, once I associate a specific channel like Raty Pekao with my payment method (for simplicity, let's call this payment method Raty Pekao) I should be redirected to this specific channel after completing the payment.

Small wins 🏆

• After creating a standard pay-by-link payment method, as a customer, I should see a list of available Tpay channels and I should be able to pick any of them while completing the order
• After creating a pay-by-link payment method, but with an associated channel, as a customer, I should not see a list of available Tpay channels and I should be redirected to this specific channel while completing the order

Tips 💡

• Existing PBL logic can be reused, as we have to use the same mechanism. The only difference is we are no longer allow the customer to choose the channel; instead, we fill the channel id based on the configuration
• There are three suggested ways the configuration should look like:
  • (@jakubtobiasz) Add a dropdown when you can select a specific channel. If none is selected, the default PBL behavior is served. If type is PBL and channel is specified, we serve the new behavior.
  • (@lchrusciel) Add a multiselect field listing all available channels; if none is picked – PBL default behavior; if one is picked – new behavior; two or more, default PBL behavior with limiting available channels to selected ones

Replaces #191 originally posted by @lchrusciel