[copp] add dhcpv6 copp rules (#1979)

What I did
Add secondary COPP config file to enable DHCP V6

Why I did it
Need to enable DHCP V6

Create separate file because:
- The new COPP rule needs to be in a separate table so that it
  could applied individually after warm reboot.
- The warm reboot has asic config checker, keeping the config
  in a separate file could avoid the checker flag it as a config
  change therefore stop warm reboot.
- Make it convenient for warm reboot finalizer to apply this
  config individually.

How I verified it
Warm reboot from a version without it to a version with it.
Warm reboot from a version with it to same version.
Cold reboot.

In all 3 cases, dhcp v6 rules are correctly installed.

Signed-off-by: Ying Xie <[email protected]>

debian/swss.install

@@ -1,3 +1,4 @@
 swssconfig/sample/netbouncer.json etc/swss/config.d
 swssconfig/sample/00-copp.config.json etc/swss/config.d
+swssconfig/sample/01-copp-dhcpv6.config.json etc/swss/config.d
 neighsyncd/restore_neighbors.py usr/bin

swssconfig/sample/01-copp-dhcpv6.config.json

@@ -0,0 +1,11 @@
+[
+    {
+        "COPP_TABLE:trap.group.dhcpv6": {
+            "trap_ids": "dhcpv6",
+            "trap_action":"trap",
+            "trap_priority":"4",
+            "queue": "4"
+        },
+        "OP": "SET"
+    }
+]