subscription: add ModifySubscription functionality

[jackchenjc]

closes #713

Simply add the following code to examples/subscribe/subscribe.go can test this new feature

var params opcua.SubscriptionParameters
params.Interval = time.Millisecond * 2000
_, err = sub.ModifySubscription(ctx, params)
if err != nil {
	log.Fatal(err)
}

[kung-foo]

These might need a lock. But it doesn't look like we have one in Subscription. There's this func (with a truly awful name):

opcua/subscription.go

Line 353 in 5aed8b4

func (s *Subscription) recreate_NeedsSubMuxLock(ctx context.Context) error {

[jackchenjc]

Do you suggest to create a new Subscription based on the current subscription and then invoke its recreate_NeedsSubMuxLock func to safely update these values?

Something like:

Suggested change

	s.params = &params
	s.itemsMu.Lock()
	sub := &Subscription{
	SubscriptionID: s.SubscriptionID,
	RevisedPublishingInterval: time.Duration(res.RevisedPublishingInterval) * time.Millisecond,
	RevisedLifetimeCount: res.RevisedLifetimeCount,
	RevisedMaxKeepAliveCount: res.RevisedMaxKeepAliveCount,
	Notifs: s.Notifs,
	items: s.items,
	params: &params,
	nextSeq: s.nextSeq,
	c: s.c,
	}
	s.itemsMu.Unlock()
	err = sub.recreate_NeedsSubMuxLock(ctx)
	if err != nil {
	return nil, err
	}