bgpd: extended-community and attrs for MAC-IP SYNC route handling

bgpd: extended-community and attrs for MAC-IP SYNC route handling

A new proxy flag has been added to the already existing NA extended
community to allow proxy advertisment of a local host by a VTEP that is
yet to indpendently establish local reachability.
Reference: draft-rbickhart-evpn-ip-mac-proxy-adv

The extendend mac-mobility sequence number needs to be synced across
the ES peers. However we cannot let a ES-peer path win over a local
path on the same ES. To accomplish that some parameters such as the
MM seq number are bubbled up from the non-best path to the local path.
This mechanism is explained further in the path-selection patch.

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>