add: dummy payment provider

[peelar]

Features:

• pages/api/dummy-pay.ts - API endpoint for dummy payment (creates a transaction)
• integrating the new payment method dummy into the existing structure (e.g. pages/api/pay.ts)
• added checkout-storefront/src/views/DummyPayment.tsx view with a simple form that lets you pay either the entirety or a fraction of the order's total price
• added the Views component in Root.tsx, as there are now three views that can be displayed: Checkout, OrderConfirmation, and DummyPayment. DummyPayment's URL is: host/checkout?order=:orderId&dummyPayment=true.

Media:

https://user-images.githubusercontent.com/44495184/193268382-4796058f-7378-4f9f-996a-3f5ccb047aca.mov

Questions:

1. How do we restrict access to the dummy payment provider? Currently, it's turned on by default. Should we add an env var like IS_DUMMY_PROVIDER_ON set to false by default?

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Name	Status	Preview	Comments	Updated
react-storefront	✅ Ready (Inspect)	Visit Preview	💬 Add your feedback	Oct 12, 2022 at 2:03PM (UTC)
saleor-app-checkout	✅ Ready (Inspect)	Visit Preview	💬 Add your feedback	Oct 12, 2022 at 2:03PM (UTC)
saleor-checkout	✅ Ready (Inspect)	Visit Preview		Oct 12, 2022 at 2:03PM (UTC)

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Name	Status	Preview	Comments	Updated
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All committers have signed the CLA.

[peelar]

How do we restrict access to the dummy payment provider?

After a conversation with @witoszekdev, I think the best way would be to include the Dummy Payment in the Dashboard configuration UI (same as the rest of the payment providers). I will add it tomorrow.

[peelar]

Added the ability to select the payment provider from the configuration. @witoszekdev @mmiszy

[peelar]

@witoszekdev merged your suggestion & rebased. Please approve & merge if all is good.

[peelar]

@mmiszy please merge if all is good