Return unsafe deserialized event data object as Option

[mickjermsurawong-stripe]

• Now that we support java language version > v8, we can use optional to support when the getting deserialized object from webhook.
• This replaces the boolean check method deserialize

Currently, the expected integration will look like this..

   Event event = Webhook.constructEvent(payload, sigHeader, secret);
   EventDataObjectDeserializer dataObjectDeserializer = event.getDataObjectDeserializer();
   if (dataObjectDeserializer.deserialize()) {
      StripeObject stripeObject = dataObjectDeserializer.getObject();
   }

The problem is rather, user might just go straight and getObject() which can be null if it is not safe.
Now

   Event event = Webhook.constructEvent(payload, sigHeader, secret);
   EventDataObjectDeserializer dataObjectDeserializer = event.getDataObjectDeserializer();
   if (dataObjectDeserializer.getObject().isPresent()) {
     StripeObject stripeObject = dataObjectDeserializer.getObject().get();
     doSomething(stripeObject);
   } 

For unsafe workflow, it's still the same. User can force deserialization or can use compatability to make the Json compatible

The original implementation and discussion in here #654
r? @ob-stripe
cc @stripe/api-libraries

[ob-stripe]

LGTM!

[mickjermsurawong-stripe]

self-approving after fixing one doc to use Optional<StripeObject> for event deserialization.
LGTM
thank you ob!