Access to full files/direct URL via Preview SDK

Currently there is the existing function fileURLForContentPreviewCacheEntryForFile which returns a valid file url for the cached document. However, these files are encrypted and this url is not of much use.

We are in need of a function similar to retrievePreviewImageForFile:(BOXFile *)file completion:(BOXImageBlock)completionBlock that returned temporary access to an unencrypted version of a cached file for external use that could be cleared once you are done with it. With respect to file permissions for the user of the app, of course.

The use case for this may be to make it an attachment to a message of some kind, hand it off to another app for editing such as Office/etc, or many other uses that we may not have yet thought of.

Start by reviewing the existing fileURLForContentPreviewCacheEntryForFile function and the retrievePreviewImageForFile:completion: API mentioned in the issue. Clarify the temporary unencrypted access lifecycle, user permission behavior, and external-use cases; done means the required Preview SDK API and its behavior are agreed and implemented.

Written by the indexing model from the issue text.