[orchagent] fix "parsePortConfig: Unknown field(mode)"

When using "mode" as defined in
https://github.com/sonic-net/sonic-buildimage/blob/master/src/sonic-yang-models/yang-models/sonic-port.yang
this warning is emitted for each interface:
```
WARNING swss#orchagent: :- parsePortConfig: Unknown field(mode): skipping ...
```

It appears this field is not needed in orchagent at all, so we need to
at least recognize it as valid then skip processing.

Warnings harm debugging and can cause wasted cycles when debugging
real issues.

Signed-off-by: Brad House (@bradh352)

orchagent/port/porthlpr.cpp

@@ -1229,6 +1229,12 @@ bool PortHelper::parsePortConfig(PortConfig &port) const
                 return false;
             }
         }
+        else if (field == PORT_MODE)
+        {
+            /* Placeholder to prevent warning. Not needed to be parsed here.
+             * Setting exists in sonic-port.yang with possible values: routed|access|trunk
+             */
+        }
         else
         {
             SWSS_LOG_WARN("Unknown field(%s): skipping ...", field.c_str());

orchagent/port/portschema.h

@@ -101,3 +101,4 @@
 #define PORT_SUPPRESS_THRESHOLD    "suppress_threshold"
 #define PORT_REUSE_THRESHOLD       "reuse_threshold"
 #define PORT_FLAP_PENALTY          "flap_penalty"
+#define PORT_MODE                  "mode"