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.rs

@@ -18,6 +18,7 @@ mod random_channel_buffer_tests;
 mod random_project_collaboration_tests;
 mod randomized_test_helpers;
 mod remote_editing_collaboration_tests;
+mod stripe_billing_tests;
 mod test_server;
 
 use language::{Language, LanguageConfig, LanguageMatcher, tree_sitter_rust};