enable AfiSafiType optionally coming from a field on a NLRI struct

[density215]

I can't impl AfiSafi and IsNlri for BasicNlri, because the relevant methods don't take &self:

pub struct BasicNlri {
    pub ty: AfiSafiType,
    pub prefix: Prefix,
    pub path_id: Option<PathId>,
}

impl routecore::bgp::nlri::afisafi::IsNlri for BasicNlri {
     fn nlri_type() -> NlriType {
         todo!()
     }   
 }

 impl routecore::bgp::nlri::afisafi::AfiSafi for BasicNlri {
     fn afi() -> routecore::bgp::types::Afi {
         todo!()
     }
     fn afi_safi() -> AfiSafiType {
         todo!()
     }
 }

This PR fixes that.