feat: Add .Response() and .Err() method for all subscriber

[GoudanWoo]

Sometimes when we call this project to subscribe to some data, we need to use select {} for timeout control. like:

solana-go/rpc/sendAndConfirmTransaction/sendAndConfirmTransaction.go

Lines 123 to 143 in 790032b

	for {
	select {
	case <-ctx.Done():
	return false, ctx.Err()
	case <-time.After(*timeout):
	return false, ErrTimeout
	case resp, ok := <-sub.Response():
	if !ok {
	return false, fmt.Errorf("subscription closed")
	}
	if resp.Value.Err != nil {
	// The transaction was confirmed, but it failed while executing (one of the instructions failed).
	return true, fmt.Errorf("confirmed transaction with execution error: %v", resp.Value.Err)
	} else {
	// Success! Confirmed! And there was no error while executing the transaction.
	return true, nil
	}
	case err := <-sub.Err():
	return false, err
	}
	}

But if there is only .Recv() method, we are unable to complete requirements such as timeout control and active cancel. So I added .Response() and .Err() to do it.

[gagliardetto]

thanks!