collab: Add billing preferences for maximum LLM monthly spend (#18948)

This PR adds a new `billing_preferences` table.

Right now there is a single preference: the maximum monthly spend for
LLM usage.

Release Notes:

- N/A

---------

Co-authored-by: Richard <richard@zed.dev>

crates/collab/src/db.rs

@@ -42,6 +42,9 @@ pub use tests::TestDb;
 
 pub use ids::*;
 pub use queries::billing_customers::{CreateBillingCustomerParams, UpdateBillingCustomerParams};
+pub use queries::billing_preferences::{
+    CreateBillingPreferencesParams, UpdateBillingPreferencesParams,
+};
 pub use queries::billing_subscriptions::{
     CreateBillingSubscriptionParams, UpdateBillingSubscriptionParams,
 };