Store Stripe Customer ID

[easyCZ]

We actually need to store the Stripe Customer ID to avoid eventual consistency on Stripe Search API.

• Store Stripe Customer ID in the DB
• Populate the DB when creating Customers
• Make it possible for the StripeCustomerID to change for a given Attribution, as a new customer is needed to enable currency switch

This issue blocks #13201

[easyCZ]

In the first phase, we'll introduce the DB schema and the DB model, and we'll populate it during the Customer Search (use it as a cache). In the subsequent phase, we'll also move the CreateStripeCustomer call over to usage and store it in the DB directly then.

[easyCZ]

This is now running behind a feature flag in production. We'll continue to monitor for the next week and if all looks good, we can remove the feature flag. Tracking flag removal in #14039

[easyCZ]

Validation so far looks great. Closing this issue.