Why I did it
Currently the config cli of dhcpv4 is may cause confusion and config of dhcpv6 is missing.
How I did it
Add dhcp_relay config cli and test cases.
config dhcp_relay ipv4 helper (add | del) <vlan_id> <helper_ip_list>
config dhcp_relay ipv6 destination (add | del) <vlan_id> <destination_ip_list>
Updated docs for it in sonic-utilities: https://github.com/sonic-net/sonic-utilities/pull/2598/files
How to verify it
Build docker-dhcp-relay.gz with and without INCLUDE_DHCP_RELAY, and check target/docker-dhcp-relay.gz.log
