Consider Notify for pgdriver with transaction signature

[vabatta]

There might be cases where

bun/driver/pgdriver/listener.go

Lines 21 to 24 in 0d441a2

	func Notify(ctx context.Context, db *bun.DB, channel, payload string) error {
	_, err := db.ExecContext(ctx, "NOTIFY ?, ?", bun.Ident(channel), payload)
	return err
	}

might be needed to run within an existing transaction, shall we consider to add this as a new method with signature?

[vmihailenco]

It might make sense to try using db bun.IDB instead...