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/stripe_client.rs

@@ -0,0 +1,33 @@
+#[cfg(test)]
+mod fake_stripe_client;
+mod real_stripe_client;
+
+use std::sync::Arc;
+
+use anyhow::Result;
+use async_trait::async_trait;
+
+#[cfg(test)]
+pub use fake_stripe_client::*;
+pub use real_stripe_client::*;
+
+#[derive(Debug, PartialEq, Eq, Hash, Clone)]
+pub struct StripeCustomerId(pub Arc<str>);
+
+#[derive(Debug, Clone)]
+pub struct StripeCustomer {
+    pub id: StripeCustomerId,
+    pub email: Option<String>,
+}
+
+#[derive(Debug)]
+pub struct CreateCustomerParams<'a> {
+    pub email: Option<&'a str>,
+}
+
+#[async_trait]
+pub trait StripeClient: Send + Sync {
+    async fn list_customers_by_email(&self, email: &str) -> Result<Vec<StripeCustomer>>;
+
+    async fn create_customer(&self, params: CreateCustomerParams<'_>) -> Result<StripeCustomer>;
+}