collab: Add billing_subscriptions table (#15448)

This PR adds a new `billing_subscriptions` table to the database, as
well as some accompanying models/queries.

In this table we store a minimal amount of data from Stripe:

- The Stripe customer ID
- The Stripe subscription ID
- The status of the Stripe subscription

This should be enough for interactions with the Stripe API (e.g., to
[create a customer portal
session](https://docs.stripe.com/api/customer_portal/sessions/create)),
as well as determine whether a subscription is active (based on the
`status`).

Release Notes:

- N/A

crates/collab/src/db.rs

@@ -45,6 +45,7 @@ use tokio::sync::{Mutex, OwnedMutexGuard};
 pub use tests::TestDb;
 
 pub use ids::*;
+pub use queries::billing_subscriptions::CreateBillingSubscriptionParams;
 pub use queries::contributors::ContributorSelector;
 pub use sea_orm::ConnectOptions;
 pub use tables::user::Model as User;