Validate free connector program enrollment with backend before showing success/error UI notifications

[ambirdsall]

After a user goes through the Stripe checkout page, Stripe simultaneously redirects them to the successUrl we provide and sends a request to a webhook of ours. For a small number of users, this webhook is not successfully processed, preventing us from recognizing them as enrolled and billing accordingly; we should not show any confirmation UI until we have verified with the Airbyte cloud backend that the webhook has been processed correctly. The initial implementation didn't account for this, and triggered the confirmation UI as soon as the user navigated to the successUrl; this needs to be updated. Lots of context in this slack thread.

So:

• when a user navigates to a page whose render invokes useFreeConnectorProgram() with the fcpEnrollmentSuccess query param, start polling the FCP program info endpoint
• if polled return value ever says hasPaymentAccountSaved is true, stop polling, hide enrollment banners, and show success UI
• if some timeout is reached, show an error popup instead

One weakness of this approach: it cannot detect errors in a scenario where users try to update the payment account of record for a workspace. We don't currently have any UI to manage payment accounts, but we'll need it: it's a table stakes feature for actually doing any billing via the saved payment accounts.