Async/Non-Blocking Feature Evaluation

I notice in the javascript API, `getBooleanValue` returns a `Promise` instead of `boolean`. Would it be possible to have an API like this in the Java case that would support non-blocking use-cases. An example would be, for example, in a Spring WebFlux service where evaluation is based on the some attributes of the current request and needs to delegate evaluation to another external service.

Start by reviewing the Java SDK evaluation API and the JavaScript API's Promise-based getBooleanValue reference. Consider the Spring WebFlux use case and external-service delegation described in the issue; done would require an agreed non-blocking Java API design and defined scope.

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