server: Auto derived ES-Import RT for EVPN Ethernet Segment route
server: Auto derived ES-Import RT for EVPN Ethernet Segment route

According to RFC7432, the ES-Import RT can be derived automatically
when using the ESI Types 1, 2 and 3.

This patch enables to derive automatically the ES-Import RT for the
given path from BgpServer's API.

Signed-off-by: IWASE Yusuke <bb80e84cc9f5a8f01385872ee89032beb5d4eba7@gmail.com>

fujita/gobgp,fujita/gobgp,a16/gobgp,osrg/gobgp,a16/gobgp,j-kato/gobgp,j-kato/gobgp,fujita/gobgp,tamihiro/gobgp,osrg/gobgp,tamihiro/gobgp,a16/gobgp,tamihiro/gobgp,a16/gobgp,a16/gobgp,fujita/gobgp,a16/gobgp,a16/gobgp,tamihiro/gobgp,osrg/gobgp,osrg/gobgp,j-kato/gobgp