ospf6d: Auth trailer CLI implementation.

ospf6d: Auth trailer CLI implementation.

Problem Statement:
==================
RFC 7166 support for OSPF6 in FRR code.

RCA:
====
This feature is newly supported in FRR

Fix:
====
Changes are done to add support for two new CLIs to configure
ospf6 authentication trailer feature.
One CLI is to support manual key configuration.
Other CLI is to configure key using keychain.

below CLIs are implemented as part of this commit. this configuration
is applied on interface level.

Without openssl:
ipv6 ospf6 authentication key-id (1-65535) hash-algo <md5|hmac-sha-256> key WORD

With openssl:
ipv6 ospf6 authentication key-id (1-65535) hash-algo <md5|hmac-sha-256|hmac-sha-1|hmac-sha-384|hmac-sha-512> key WORD

With keychain support:
ipv6 ospf6 authentication keychain KEYCHAIN_NAME

Running config for these command:

frr# show running-config
Building configuration...

Current configuration:
!
interface ens192
 ipv6 address 2001:DB8:1::2/64
 ipv6 ospf6 authentication key-id 10 hash-algo hmac-sha-256 key abhinay
!
interface ens224
 ipv6 address 2001:DB8:2::2/64
 ipv6 ospf6 authentication keychain abhinay
!

Risk:
=====
Low risk

Tests Executed:
===============
Have executed the combination of commands.

Signed-off-by: Abhinay Ramesh <rabhinay@vmware.com>