Bgp4MpStateChange and Bgp4MpMessage have different field name for peer_ip

[ties]

It seems like Bgp4MpStateChange and Bgp4MpMessage have an inconsistent name for the peer ip address, while in the MRT RFC (RFC 6396) the naming seems to be identical.

I think it would be nice to deprecate one of the names and make them consistent;

                bgpkit_parser::models::Bgp4MpEnum::StateChange(state_change) => {
                    (state_change.peer_addr, state_change.peer_asn)
                }
                bgpkit_parser::models::Bgp4MpEnum::Message(inner) => {
                    (inner.peer_ip, inner.peer_asn)
                }