sqldb: add table to track custom SQL migrations

This commit adds the migration_tracker table which we'll use to track if
a custom migration has already been done.

sqldb/sqlc/models.go

@@ -91,3 +91,8 @@ type InvoiceSequence struct {
 	Name         string
 	CurrentValue int64
 }
+
+type MigrationTracker struct {
+	Version       int32
+	MigrationTime time.Time
+}