collab: Add orb_customer_id to billing_customers (#36310)

This PR adds an `orb_customer_id` column to the `billing_customers`
table.

Release Notes:

- N/A

crates/collab/migrations/20250816133027_add_orb_customer_id_to_billing_customers.sql

@@ -0,0 +1,2 @@
+alter table billing_customers
+    add column orb_customer_id text;