collab: Add kind and period start/end timestamps to billing_subscriptions (#28796)

This PR updates the `billing_subscriptions` table with some new columns

- `kind` - The kind of the description (used to denote Zed Pro vs
existing)
- `stripe_current_period_start` - The Stripe timestamp of when the
subscriptions current period starts
- `stripe_current_period_end` - The Stripe timestamp of when the
subscriptions current period ends

Release Notes:

- N/A

Co-authored-by: Mikayla <mikayla@zed.dev>

crates/collab/migrations/20250415164141_add_kind_and_period_to_billing_subscriptions.sql

@@ -0,0 +1,4 @@
+alter table billing_subscriptions
+    add column kind text,
+    add column stripe_current_period_start bigint,
+    add column stripe_current_period_end bigint;