bgp: T6032: add EVPN MAC-VRF Site-of-Origin support

In some EVPN deployments it is useful to associate a logical VTEP's Layer 2
domain (MAC-VRF) with a Site-of-Origin "site" identifier. This provides a BGP
topology-independent means of marking and import-filtering EVPN routes
originated from a particular L2 domain. One situation where this is valuable
is when deploying EVPN using anycast VTEPs

set protocols bgp address-family l2vpn-evpn mac-vrf soo