POC - Refactoring settings (2)

[wjrosa]

Fixes STRIPE-<linear_issue_id>
Fixes #<github_issue_id>

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[wjrosa]

@woocommerce/quark I tried to update the settings retrieval code with the main WC_Stripe class (as @diegocurbelo suggested during the meetup). See some of the results here. That did not work because some classes we instantiate inside WC_Stripe (like WC_Stripe_Payment_Request) require the settings. That was causing an infinite loop. The solution would be to access the options on those classes directly, but that would leave us with two different standards anyway.

Today, I came up with this solution here instead. Although it introduces a new class, it uses the singleton pattern and also guarantees that settings are always fresh when updated. What do you think?

This is just a POC PR. If you think the approach is nice, we can work on this later with multiple smaller PRs.