Adding a get sync getItems method

[cunhazera]

Fixes #109

Commentary:

• It would be little harder to have a toSync method, because I'd need to return an instance of DeliveryClient so the items should be stored in a field, so then I could have an deliveryClient.getItems(Class.class).toSync(). If I did this, the DeliveryClient class would need a generic type at the class level, so that would break a lot of old code. Anyway, if anyone has a better idea we can talk about, to have a toSync method.

[Simply007]

This pull request only covers only the getItems method, but there is a bunch of remaining in the delivery client (i.e. getTypes).

I would like to avoid doubling the method count in the delivery client. I would appreciate a unified approach.
It might be possible to inherit from CompletionStage and extend it with a method called toSync. This would not break any compatibility, because of inheritance and it would allow having nice and fluent syntax as described in #109 (comment).

According to the tests, I would like to not have current tests modified and just add new ones, because the functionality is also just added, not modified.

Mind the Readme - there should be the mention of the functionality too.

[Simply007]

#112 (comment)

[Simply007]

@cunhazera - do you have any update on this?

[Simply007]

Hi!

This pull request has gone quiet. 👻
It’s been a while since the last update here.

If we missed anything on this issue or if you want to keep it open, feel free to do so.

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