…n disk for file (elastic#79698)

In elastic#79371 we fixed a bug where cache files were not created 
as sparse files on Windows platforms because the wrong 
options were used when creating the files for the first time. 
This bug got unnoticed as we were lacking a way to retrieve 
the exact number of bytes allocated for a given file on disk.

This commit adds a FileSystemNatives.allocatedSizeInBytes(Path) 
method for that exact purpose (only implemented for Windows 
for now) and a test in CacheFileTests that would fail on 
Windows if the cache file is not sparse.

Relates elastic#79371