Add dns.edns.Option.to_generic() (#1145)

Add dns.edns.Option.to_generic() (#1145)

* Add dns.edns.Option.to_generic()

Converts an EDNS option represented by a custom class into an equivalent
option represented by the generic option class.  This is similar to the
existing dns.rdata.Rdata.to_generic() method.

Also, adds a specialization to the existing dns.rdata.Rdata.to_generic()
method for GenericRdata, to avoid extra work for applications that want
to convert all rdata to generic form.

* Fix typing issue.

* Fix typing again.