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/migrations/20241009190639_add_billing_preferences.sql

@@ -0,0 +1,8 @@
+create table if not exists billing_preferences (
+    id serial primary key,
+    created_at timestamp without time zone not null default now(),
+    user_id integer not null references users(id) on delete cascade,
+    max_monthly_llm_usage_spending_in_cents integer not null
+);
+
+create unique index "uix_billing_preferences_on_user_id" on billing_preferences (user_id);