Why move Closeable to the implementation? Shouldn't all EventSourceManager implementations be Closeable?

EventSourceManager is just the API that user should see from the init method. Should not be aware that it is actually closeable, it's an implementation detail in the background.

Well, if we had several implementations of the interface, all of them should be closeable. So, to me, Closeable should be part of the contract of the EventSourceManager interface. Otherwise, if we push your logic to its conclusion, then we shouldn't even have a interface because we only have one implementation and that implementation needs to be closeable for the SDK to work properly. 😄
If we don't expect any other implementations, I don't think we need to worry about having an interface just for the sake of hiding some things away. Worse, having an interface implies that we potentially expect several implementations, which isn't really the case. It makes things more complex without much benefit.

I think it makes sense, it's basically interface segregation principle:
https://en.wikipedia.org/wiki/Interface_segregation_principle
The intended users should not see the other methods if we don't intend it for them.
They should not care if there is an other implementation or not, this is what we provide, it should be used.

The benefit is clear user facing interface. But without to much added complexity (close to 0).

maybe @lburgazzoli or @adam-sandor , what do you think?

discussed with @metacosm . Will merge it now, and will do subsequent PRs, if we find a better structure/way.