collab: Introduce StripeClient trait to abstract over Stripe interactions (#31615)

This PR introduces a new `StripeClient` trait to abstract over
interacting with the Stripe API.

This will allow us to more easily test our billing code.

This initial cut is small and focuses just on making
`StripeBilling::find_or_create_customer_by_email` testable. I'll follow
up with using the `StripeClient` in more places.

Release Notes:

- N/A

crates/collab/src/api/billing.rs

@@ -344,6 +344,7 @@ async fn create_billing_subscription(
         stripe_billing
             .find_or_create_customer_by_email(user.email_address.as_deref())
             .await?
+            .try_into()?
     };
 
     let success_url = format!(