Add a small abstraction layer for xattr

[Schamper]

Currently we just return the raw implementation specific object for xattr on filesystems where we have implemented that (which I think is only extfs):

dissect.target/dissect/target/filesystems/extfs.py

Lines 144 to 148 in d0fa985

	def attr(self) -> Any:
	return self._resolve().entry.xattr
	def lattr(self) -> Any:
	return self.entry.xattr

We should add a small abstraction layer to unify this across filesystems, especially once we add more implementations: fox-it/dissect.xfs#37

Would be nice if this also took into account NTFS attributes. Maybe a simple "dataclass" with a name: str, value: bytes and a raw: Any of the implementation specific object?