add @Nullable annotation to specify union with null

HDFS now includes a class (HdfsFileStatus) with a field (symlink) whose type is byte[] but which is null for non-symlinks. For most other types, we could annotate this with something like @Union(

) to declare that the field may be null. But there is unfortunately no way to refer to the class of byte[] in an annotation. So, instead, I propose to add an annotation that permits the specification of an arbitrary schema. In this case, the annotation would then be @AvroSchema("[\"null\",\bytes\"").