Add "dest_type" option to name_new/firstupdate/update

[JeremyRand]

Is your feature request related to a problem? Please describe.

When the send RPC method generates an output address on behalf of the user (i.e. the change address), it supports both an explicit change_address option (user specifies the address) and an implicit change_type option (user only specifies the address type; a change address is generated automatically of the requested type). The name RPC methods also generate an output address on behalf of the user (the name output address), but they only support specifying an explicit destination address, not an address type. This is unfortunate.

Describe the solution you'd like

Add a dest_type field to the name RPC methods, which influences the name output address in the same way that the change_type field influences the change address in the send RPC method. This improves parity between currency and name transactions.

Describe alternatives you've considered

N/A.

Additional context

N/A.