Investigation: Making express checkout work even when cards are not enabled

[annemirasol]

Is your feature request related to a problem? Please describe.

When credit cards are not enabled, using Apple Pay/Google Pay in an order will result in an Invalid payment method error. This is because checkout fails WooCommerce validation due to the stripe payment gateway not being available.

When the payment gateway validation part is forcibly skipped (e.g. by commenting it out), payment succeeds and the order pushes through without any problem (AFAICT).

This issue was created in order to revisit the decision made in #3589 to hide express checkout when cards are disabled.

Describe the solution you'd like

We should investigate, discuss and document the following points:

1. Do we want to make express checkout work, even when credit cards are not enabled? Are there practical use cases for such a setup?
2. Should we be using stripe as the gateway ID for credit card payments? Should we be using stripe as the gateway ID for express checkout?
3. Can we safely skip gateway validation for express payment methods?