sqldb: add invoice events SQL schema

sqldb/sqlc/migrations/000003_invoice_events.down.sql

@@ -0,0 +1,9 @@
+DROP TABLE IF EXISTS invoice_events;
+
+DROP INDEX IF EXISTS invoice_events_created_at_idx;
+DROP INDEX IF EXISTS invoice_events_invoice_id_idx;
+DROP INDEX IF EXISTS invoice_events_htlc_id_idx;
+DROP INDEX IF EXISTS invoice_events_set_id_idx;
+DROP INDEX IF EXISTS invoice_events_event_type_idx;
+
+DROP TABLE IF EXISTS invoice_event_types;