Support global copy_to feature

[dadoonet]

As seen in #97, we don't support a global copy_to feature on attachment type but only on generated sub fields. It could be handy to copy a full attachment field to another field:

{
    "person": {
        "properties": {
            "file": {
                "type": "attachment",
                "copy_to": "copy"
            },
            "copy": {
              "type": "attachment"
            }
        }
    }
}

[dadoonet]

Well. May be this feature does not really make sense. At the end of the day, we might want to copy_to a generated field (file extracted content itself, metadata...) but copying the BASE64 content to another field actually does not add much value.

Closing.