Allow serialization groups to be accessible within event subscriber callbacks.

[craigmarvelley]

I'm using event subscribers to add additional data post-serialize. I'd like to only add certain properties if a particular exclusion strategy group is active, but can't see a way to access that information from within the callback. Assuming I haven't missed something, would it be possible to add this functionality, perhaps by making it available from the event? I'd make a PR if so.

[alex88]

👍

[fieg]

+1

It's possible to access the GroupsExclusionStrategy with $event->getContext()->getExclusionStrategy(), but the groups property is protected and not exposed via a public method.

Also relates to #197 and #129 if you ask me.

[goetas]

see #554