Add Async Flavor Interfaces

e.g. client.folder(folder_id='0').upload_async('/path/to/file'), which returns a promise.

Start by locating the client.folder(folder_id='0') entry point and the existing upload operation in the SDK. Determine how broadly async flavor interfaces are intended to apply and how a returned promise should be represented. Done means the agreed async interface scope is implemented consistently and its behavior is covered by tests.

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