Add bank redirect support (EPS, Sofort) to Payment Element Gateway

Problem/Motivation

The PaymentElementGateway has been refactored to support additional payment method types in 📌 Refactor Stripe payment element for additional payment method types Fixed . At the same time, this change forces us to implement specific plugins for every supported payment method. (see https://git.drupalcode.org/project/commerce_stripe/-/commit/bf118321d3fb...).

We'll need ✨ Add the option to not setup payment methods for future usage Fixed in order to actually being able to select the new method(s), but we can nevertheless implement the plugin independently from that.

One main point to discuss before we start, if we do need a dedicated implementatin per bank redirect gateway (like EPS, Sofort, etc), or if they all can be treated equally by implementing a generic plugin. I believe, we need dedicated ones though, but at least a common abstract base class might help us

Proposed resolution

Implement the plugin.

Remaining tasks

Before we start:

• Dedicated plugin per payment method vs generic bank redirect plugin - if first: does a common base class for them make sense?
• Do we need any extra fields?
• Any special treatment necessary?
• Link to relevant parts of the Stripe API doc