Implement a dummy payment gateway

[DanReyLop]

This gateway won't do any actual requests to the actual payment partner, but it will:

• Register itself as a WooCommerce gateway.
• Have a minimal Settings screen with some dummy fields.
• Simulate (i.e always accept) payments in the checkout page.

Keep in mind that we'll need to keep the PHP file structure and class naming in accordance with WooCommerce Core code styles, since we want this to be a feature plugin.

[dechov]

Instead of or in addition to dummy fields, we should have "Enabled", "Title", and "Description" settings, which I believe are expected for payment gateway instances [doc], and are common controls for checkout appearance.