feat: rate limit adding new payment methods by user

[dkoo]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Building on #3678, improves rate limiting for adding payment methods in My Account.

How to test the changes in this Pull Request:

1. Check out this branch and enable/connect at least two different payment gateways.
2. In wp-config.php, define NEWSPACK_CHECKOUT_RATE_LIMIT as something long enough for manual testing like 60 or 90.
3. As a reader, log in and visit /my-account/add-payment-method to add a new payment method.
4. Add a new payment method for Stripe using a test Stripe card which will be declined. Confirm that you get a "card was declined" error.
5. Within the number of seconds defined in step 2, attempt to submit the same or a different card number and confirm that you get an error message:

6. Within the number of seconds defined in step 2, attempt to submit a new payment method with a different payment gateway and confirm that you get the same error message again.
7. Wait until after the time limit has elapsed, then try to add a valid card number once more with either payment gateway and confirm that it succeeds.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?