Add PPPoE Plugin

Add PPPoE Plugin

Supports 64K PPPoE sessions

This plugin adds three graph nodes:
1) pppoe-input for PPPoE decapsulation
2) pppoe-encap for PPPoE encapsulation
3) pppoe-tap-dispatch for control plane process

Below is the configuration to make PPPoE CP and DP work:
vim /etc/vpp/startup.conf
tuntap {
  enable
  ethernet
  name newtap
}

create pppoe tap tap-if-index 1

//Configure it after a subscriber's PPPoE discovery and PPP link establishment succeeds:
create pppoe session client-ip 100.1.2.1 session-id 1 client-mac 00:11:01:00:00:01

show pppoe fib
show pppoe session

Change-Id: I73e724b6bf7c3e4181a9914c5752da1fa72d7e60
Signed-off-by: Hongjun Ni <hongjun.ni@intel.com>