[dhcp-relay] Add support for dhcp_relay config cli (sonic-net#13373)

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

Author: yaqiangz

dockers/docker-dhcp-relay/cli-plugin-tests/conftest.py

@@ -10,6 +10,11 @@ def mock_cfgdb():
             'Vlan1000': {
                 'dhcp_servers': ['192.0.0.1']
             }
+        },
+        'DHCP_RELAY': {
+            'Vlan1000': {
+                'dhcpv6_servers': ['fc02:2000::1']
+            }
         }
     }