Add capability from rfc8950.

Just the basic parser, still needs to be integrated into the rest of the
code for the OPEN message parsing, and then the rest of the MP
Reach/Unreach NLRI parsing needs to be added.

bgpd/src/server/peer.rs

@@ -1276,6 +1276,7 @@ where
                 for attr in &u.path_attributes {
                     match attr {
                         PathAttribute::MPReachNLRIPathAttribute(nlri) => {
+                            // TODO: Determine which AFI/SAFI this update corresponds to.
                             let nexthop_res = nlri.clone().nexthop_to_v6();
                             // TODO: How do we pick whether to use the global or LLNH?
                             if let Some((global, _llnh_opt)) = nexthop_res {
@@ -1287,6 +1288,7 @@ where
                             }
                         }
                         PathAttribute::MPUnreachNLRIPathAttribute(nlri) => {
+                            // TODO: Determine which AFI/SAFI this update corresponds to.
                             self.process_withdrawals(nlri.nlris.clone())?;
                         }
                         _ => {}