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/fake_stripe_client.rs

@@ -0,0 +1,47 @@
+use std::sync::Arc;
+
+use anyhow::Result;
+use async_trait::async_trait;
+use collections::HashMap;
+use parking_lot::Mutex;
+use uuid::Uuid;
+
+use crate::stripe_client::{CreateCustomerParams, StripeClient, StripeCustomer, StripeCustomerId};
+
+pub struct FakeStripeClient {
+    pub customers: Arc<Mutex<HashMap<StripeCustomerId, StripeCustomer>>>,
+}
+
+impl FakeStripeClient {
+    pub fn new() -> Self {
+        Self {
+            customers: Arc::new(Mutex::new(HashMap::default())),
+        }
+    }
+}
+
+#[async_trait]
+impl StripeClient for FakeStripeClient {
+    async fn list_customers_by_email(&self, email: &str) -> Result<Vec<StripeCustomer>> {
+        Ok(self
+            .customers
+            .lock()
+            .values()
+            .filter(|customer| customer.email.as_deref() == Some(email))
+            .cloned()
+            .collect())
+    }
+
+    async fn create_customer(&self, params: CreateCustomerParams<'_>) -> Result<StripeCustomer> {
+        let customer = StripeCustomer {
+            id: StripeCustomerId(format!("cus_{}", Uuid::new_v4()).into()),
+            email: params.email.map(|email| email.to_string()),
+        };
+
+        self.customers
+            .lock()
+            .insert(customer.id.clone(), customer.clone());
+
+        Ok(customer)
+    }
+}

crates/collab/src/stripe_client/real_stripe_client.rs

@@ -0,0 +1,74 @@
+use std::str::FromStr as _;
+use std::sync::Arc;
+
+use anyhow::{Context as _, Result};
+use async_trait::async_trait;
+use stripe::{CreateCustomer, Customer, CustomerId, ListCustomers};
+
+use crate::stripe_client::{CreateCustomerParams, StripeClient, StripeCustomer, StripeCustomerId};
+
+pub struct RealStripeClient {
+    client: Arc<stripe::Client>,
+}
+
+impl RealStripeClient {
+    pub fn new(client: Arc<stripe::Client>) -> Self {
+        Self { client }
+    }
+}
+
+#[async_trait]
+impl StripeClient for RealStripeClient {
+    async fn list_customers_by_email(&self, email: &str) -> Result<Vec<StripeCustomer>> {
+        let response = Customer::list(
+            &self.client,
+            &ListCustomers {
+                email: Some(email),
+                ..Default::default()
+            },
+        )
+        .await?;
+
+        Ok(response
+            .data
+            .into_iter()
+            .map(StripeCustomer::from)
+            .collect())
+    }
+
+    async fn create_customer(&self, params: CreateCustomerParams<'_>) -> Result<StripeCustomer> {
+        let customer = Customer::create(
+            &self.client,
+            CreateCustomer {
+                email: params.email,
+                ..Default::default()
+            },
+        )
+        .await?;
+
+        Ok(StripeCustomer::from(customer))
+    }
+}
+
+impl From<CustomerId> for StripeCustomerId {
+    fn from(value: CustomerId) -> Self {
+        Self(value.as_str().into())
+    }
+}
+
+impl TryFrom<StripeCustomerId> for CustomerId {
+    type Error = anyhow::Error;
+
+    fn try_from(value: StripeCustomerId) -> Result<Self, Self::Error> {
+        Self::from_str(value.0.as_ref()).context("failed to parse Stripe customer ID")
+    }
+}
+
+impl From<Customer> for StripeCustomer {
+    fn from(value: Customer) -> Self {
+        StripeCustomer {
+            id: value.id.into(),
+            email: value.email,
+        }
+    }
+}