Support for EVPN Multi-Homing (EVPN-MH)

[jbemmel]

See https://docs.frrouting.org/en/stable-8.0/bgp.html#evpn-multihoming for FRR

What do people prefer:

1. New module (evpn-mh)
2. New plug-in or
3. make it part of the existing EVPN module?

[ipspace]

Looks like all you need is the same ESI across multiple interfaces. If that's the case, then we can add evpn.esi link attribute to the EVPN module. Don't want to go down any compatibility rabbit trails though.