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/rpc.rs

@@ -4039,7 +4039,8 @@ async fn get_llm_api_token(
         } else {
             let customer_id = stripe_billing
                 .find_or_create_customer_by_email(user.email_address.as_deref())
-                .await?;
+                .await?
+                .try_into()?;
 
             find_or_create_billing_customer(
                 &session.app_state,