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/tests/stripe_billing_tests.rs

@@ -0,0 +1,60 @@
+use std::sync::Arc;
+
+use pretty_assertions::assert_eq;
+
+use crate::stripe_billing::StripeBilling;
+use crate::stripe_client::FakeStripeClient;
+
+fn make_stripe_billing() -> (StripeBilling, Arc<FakeStripeClient>) {
+    let stripe_client = Arc::new(FakeStripeClient::new());
+    let stripe_billing = StripeBilling::test(stripe_client.clone());
+
+    (stripe_billing, stripe_client)
+}
+
+#[gpui::test]
+async fn test_find_or_create_customer_by_email() {
+    let (stripe_billing, stripe_client) = make_stripe_billing();
+
+    // Create a customer with an email that doesn't yet correspond to a customer.
+    {
+        let email = "user@example.com";
+
+        let customer_id = stripe_billing
+            .find_or_create_customer_by_email(Some(email))
+            .await
+            .unwrap();
+
+        let customer = stripe_client
+            .customers
+            .lock()
+            .get(&customer_id)
+            .unwrap()
+            .clone();
+        assert_eq!(customer.email.as_deref(), Some(email));
+    }
+
+    // Create a customer with an email that corresponds to an existing customer.
+    {
+        let email = "user2@example.com";
+
+        let existing_customer_id = stripe_billing
+            .find_or_create_customer_by_email(Some(email))
+            .await
+            .unwrap();
+
+        let customer_id = stripe_billing
+            .find_or_create_customer_by_email(Some(email))
+            .await
+            .unwrap();
+        assert_eq!(customer_id, existing_customer_id);
+
+        let customer = stripe_client
+            .customers
+            .lock()
+            .get(&customer_id)
+            .unwrap()
+            .clone();
+        assert_eq!(customer.email.as_deref(), Some(email));
+    }
+}