[GCU] Marking fields under MIRROR_SESSION as create-only (sonic-net#2104)

What I did
Fix sonic-net#2068

Changes to fields under MIRROR_SESSION using GCU are not reflected and removal will result in orchagent crash unless each key is deleted and added back. This means the fields are create-only.

How I did it
Mark the fields under MIRROR_SESSION as create-only

How to verify it
unit-test

Author: wen587

generic_config_updater/patch_sorter.py

@@ -589,6 +589,7 @@ def __init__(self, path_addressing):
                 ["BGP_MONITORS", "*", "local_addr"],
                 ["BGP_MONITORS", "*", "nhopself"],
                 ["BGP_MONITORS", "*", "rrclient"],
+                ["MIRROR_SESSION", "*", "*"],
             ],
             path_addressing)
 

tests/generic_config_updater/patch_sorter_test.py

@@ -1033,6 +1033,15 @@ def test_hard_coded_create_only_paths(self):
                     "nhopself": "0",
                     "rrclient": "0"
                 }
+            },
+            "MIRROR_SESSION": {
+                "mirror_session_dscp": {
+                    "dscp": "5",
+                    "dst_ip": "2.2.2.2",
+                    "src_ip": "1.1.1.1",
+                    "ttl": "32",
+                    "type": "ERSPAN"
+                }
             }
         }
         expected = [
@@ -1058,6 +1067,11 @@ def test_hard_coded_create_only_paths(self):
             "/BGP_MONITORS/5.6.7.8/name",
             "/BGP_MONITORS/5.6.7.8/nhopself",
             "/BGP_MONITORS/5.6.7.8/rrclient",
+            "/MIRROR_SESSION/mirror_session_dscp/dscp",
+            "/MIRROR_SESSION/mirror_session_dscp/dst_ip",
+            "/MIRROR_SESSION/mirror_session_dscp/src_ip",
+            "/MIRROR_SESSION/mirror_session_dscp/ttl",
+            "/MIRROR_SESSION/mirror_session_dscp/type",
         ]
 
         actual = self.validator._get_create_only_paths(config)