API to add vrf links is missing

[timoschwarzer]

The library is missing an API to add a link with InfoKind::Vrf. It would be great if that could be added to LinkAddRequest, or even make all the request functions name(), link_info(), append_nla(), up() etc) public.

[cathay4t]

The LinkAddRequest::message_mut() could be used as temporary workaround. But yes, that feature should be served.

I don't have time for this, seeking help.

[daniel-noland]

Thanks for marking this as a good first issue, this helped me get started on this code base.

I think I did it correctly in #59. Let me know if you want any changes.