bgpd: attr changes for EAD routes

bgpd: attr changes for EAD routes

Add ESI as an inline attribute field along with the other EVPN
attributes. This may be re-worked when the rest of the EVPN
attributes find a new home.

Some cleanup has been done to get rid of stale/unused references
to ESI. And also to consolidate duplicate definitions of ES ID
types.

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