ospf6d: ASBR Summarisation feature implementation

ospf6d: ASBR Summarisation feature implementation

Feature Implementation.
========================
This feature will help in advertising the External LSAs with aggregation.
The commands allow us to tune the advertisement with different parameters
as mentioned in the CLI List below.
It can also help in case we do not want to advertise any prefix with the
no-advertise option.

New CLIs added:
===============
summary-address X:X::X:X/M$prefix [tag (1-4294967295)] [{metric (0-16777215) | metric-type (1-2)}]
no summary-address X:X::X:X/M$prefix [tag (1-4294967295)] [{metric (0-16777215) | metric-type (1-2)}]
summary-address X:X::X:X/M$prefix no-advertise
no summary-address X:X::X:X/M$prefix no-advertise
aggregation timer (5-1800)
no aggregation timer (5-1800)
show ipv6 ospf6 summary-address [detail$detail] [json]
debug ospf6 lsa aggregation

CAT RUN:
========
QE to add test scripts

Signed-Off-by: Mobashshera Rasool <mrassol@vmware.com>