Ada: implement extended attributes

[onnozweers]

The dCache API supports extended attrbutes. These can probably be used to store metadata with files. We can implement them in Ada.

The /namespace API call allows setting and removing them:

If action is ‘rm-xattr’ then extended attributes of a file or directory are removed as given by the ‘names’ item. The ‘names’ value is either a string or an array of strings.
If action is ‘set-xattr’ then extended attributes are created or modified. The optional ‘mode’ item controls whether to create a new attribute (CREATE), to modify an existing attribute (MODIFY), or to assign the value by either creating a new attribute or modifying an existing attribute (EITHER). EITHER is the default mode. The ‘attributes’ item value is a JSON Object with the new attributes,where the JSON Object’s key is the attribute name and the corresponding JSON Object’s value is this attribute’s value.