collab: Keep track of last seen Stripe event for each record (#15480)

This PR improves our Stripe event handling by keeping track of the last
event we've seen for each record.

The `billing_customers` and `billing_subscriptions` tables both have a
new `last_stripe_event_id` column. When we apply an event to one of
these records, we store the event ID that was applied.

Then, when we are going through events we can ignore any event that has
an ID that came before the `last_stripe_event_id` (based on the
lexicographical ordering of the IDs).

Release Notes:

- N/A

crates/collab/migrations/20240730122654_add_last_stripe_event_id.sql

@@ -0,0 +1,2 @@
+ALTER TABLE billing_customers ADD COLUMN last_stripe_event_id TEXT;
+ALTER TABLE billing_subscriptions ADD COLUMN last_stripe_event_id TEXT;