Class EpmsSubscriptionUpdateHandler

When updating EPMS subscription, it's done as a two-step process:

1. Create a new payment source.
2. Update the subscription with the new payment source.

This class handles the storage and retrieval of the payment source and subscription UUIDs, as well as updating the subscription with the new payment source UUID. It's necessary to store the data in session storage, because after the payment source is created, it might require a redirect to the payment app, 3DS-page, PayPal page, etc., so after supporter is redirected back, we need to update the subscription with the new payment source UUID if it's active.