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/lib.rs

@@ -9,6 +9,7 @@ pub mod migrations;
 pub mod rpc;
 pub mod seed;
 pub mod stripe_billing;
+pub mod stripe_client;
 pub mod user_backfiller;
 
 #[cfg(test)]