[202012] [DHCP_RELAY] [IPv6] Inconsistent CLI usage for add/del/show of dhcpv6_server addresses

[vivekrnv]

Description

Currently, this is how dhcpv6 address are configured for different branches
​
For IPv4 this cli would work for all branches:  config vlan dhcp_relay add <vlan_id> <dhcp_relay_destination_ip>

For IPv6:

for the latest 202012 config has to be manually added and do a "systemctl restart dhcp_relay.service" after adding the config

"VLAN": {
        "Vlan30": {
            "dhcpv6_servers": [
                "2000:1::2",
                "2000:0:1:1::2"
            ]
        }
    }, 

For the latest 202111 & 202205 This cli is enough: config vlan dhcp_relay add <vlan_id> <ipv6_addresses>

For master, this config has to be added and followed by a restart of systemctl restart dhcp_relay.service 

  "DHCP_RELAY": {
        "Vlan484": {
            "dhcpv6_servers": [
                "2000:1::2",
                "2000:0:1:1::2"
            ]
        }
    }, 

For master, it's understandable since a new config is required. For 202012, after the DHCP_RELAY became an app extension, the config/show cli was not updated and the add/del config cli won't work and the "show vlan brief" doesn't show the dhcpv6 addresses.

Describe the results you expected:

202012 cli can be made consistent