Why I did it
Currently the show and clear cli of dhcp_relayis may cause confusion.
How I did it
Add doc for it: [doc] Add docs for dhcp_relay show/clear cli sonic-utilities#2649
Add dhcp_relay config cli and test cases.
show dhcp_relay ipv4 helper
show dhcp_relay ipv6 destination
show dhcp_relay ipv6 counters
sonic-clear dhcp_relay ipv6 counters
How to verify it
Unit test all passed