Add an option to generate API calls using a DbxRequest wrapper

[jonamireh]

Summary

This PR adds DbxRequest to the public API and adds an optional parameter to the Java Stone backend on using it to generate additional methods that wrap the synchronous methods. This allows callers to capture the API request as an object

DbxRequest is functionally identical to java.util.Callable; we intend on using as a namespace for Kotlin extensions but don't want to pollute the Callable with those extensions so we're introducing a new type.

Newly generated methods with be suffixed with *Request and apply to all method types (RPC, download & upload) where they return the result (if any) or the DbxDownloader/DbxUploader class. For now, no changes to DbxDownloader/DbxUploader.

Test Plan

New tests pass

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