Add managed field to the dns_record object.
Add managed field to the dns_record object.

So people can know when it's a record set by Netlify or by themselves.

Signed-off-by: David Calavera <b9b0c8e7f90ecf337f6797c4d2e0f1dd0dbc0324@gmail.com>

netlify/ruby-client,netlify/ruby-client