collab: Add a Cents type (#18935)

This PR adds a new `Cents` type that can be used to represent a monetary
value in cents.

This cuts down on the primitive obsession we were using when dealing
with money in the billing code.

Release Notes:

- N/A

crates/collab/src/lib.rs

@@ -1,5 +1,6 @@
 pub mod api;
 pub mod auth;
+mod cents;
 pub mod clickhouse;
 pub mod db;
 pub mod env;
@@ -20,6 +21,7 @@ use axum::{
     http::{HeaderMap, StatusCode},
     response::IntoResponse,
 };
+pub use cents::*;
 use db::{ChannelId, Database};
 use executor::Executor;
 pub use rate_limiter::*;