Allow creation of FileAttributeView for non-existing files

[overheadhunter]

Currently, CryptoFS throws an NoSuchFileException (wrapped in an UncheckedIOException) when attempting to get a FileAttributeView. However this violates the contract.

We must not check if the file exists until any operations on the view get invoked. Therefore we need to refactor our CryptoFileAttributeViewProvider.

Test (should not throw any exception):

Path file = cryptoFileSystem.getPath("nonExistingFile.txt");
BasicFileAttributeView attrView = Files.getFileAttributeView(file, BasicFileAttributeView.class);
Files.createFile(file);
BasicFileAttributes attrs = attrView.readAttributes();