UserDefinedAttributeView already facilitates extended attributes to store key/value pairs in file metadata. All keys are prefixed with "user." before being stored as xattr. Conversely, only xattr with this prefix are being read from a file.

While this allows associating KV pairs with files from within Java, it does not allow to write Java applications that need access to attributes using arbitrary keys such as:

* flags set by the OS
* labels, the user applied via the file manager
* metadata associated by third party software

Since access to extended attributes is already implemented, I propose to:

1. expose this API using a new interface ExtendedAttributeView and move existing code to access xattr to a corresponding implementation
2. refactor UserDefinedFileAttributeView to wrap an ExtendedAttributeView and apply the "user." prefix