[java-storage] Allow bypassing OS page cache with TransferManager (O_DIRECT)

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**Is your feature request related to a problem? Please describe.**
I need to download files from GCS without going through the OS page cache.

Downloading large files through the OS page cache causing performance problems for my application.

**Describe the solution you'd like**
I'd like to be able to use O_DIRECT to write directly to disk, ideally being able to use https://github.com/smacke/jaydio

**Describe alternatives you've considered**
None.

**Additional context**
None.

Start by locating the java-storage TransferManager download entry points and reviewing the jaydio and O_DIRECT option described in the issue. Clarify the API and platform constraints, then define completion as downloads bypassing the OS page cache without regressing existing TransferManager behavior.

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