Add support for JDK's CompletableFuture into the API as part of the Reactor.io API, add docs

Add API support into reactor.io module for JDK's standard [CompletableFuture](https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/concurrent/CompletableFuture.html) using same technique as distributed rate limiting example's integration with BucketJ in [this diff](https://github.com/confluentinc/parallel-consumer/compare/master...astubbs:features/rate-limiting#diff-9e6185103da281ca730d012e8c67ca198d099f103a60c62ce81dc7ad5a751275R81).
- [ ] Add as convenience method to API in order to more obviously demonstrate the capability.
- [ ] Add docs