Support dot in the additional information of the PFC watchdog event in the yang model (sonic-net#18235)

### Why I did it

Support dot (`.`) in the `additional_info` field in the PFC watchdog event in the yang model.

The `additional_info` field was introduced to represent diagnosis information when a PFC storm is detected, which can include fragments.

Signed-off-by: Stephen Sun <[email protected]>

### How I did it

#### How to verify it

Unit test.

Author: stephenxs

src/sonic-yang-models/tests/yang_model_tests/tests/sonic-events-swss.json

@@ -40,6 +40,9 @@
 	"SONIC_EVENTS_SWSS_PFC_STORM_VALID_WITH_ADDITIONAL_INFO": {
 		"desc": "VALID IF_STATE EVENT."
 	},
+	"SONIC_EVENTS_SWSS_PFC_STORM_VALID_WITH_ADDITIONAL_INFO_FRAGMENT": {
+		"desc": "VALID IF_STATE EVENT."
+	},
 	"SONIC_EVENTS_SWSS_PFC_STORM_WITH_INVALID_ADDITIONAL_INFO_1": {
 	    "desc": "PFC_STORM_EVENT_INCORRECT_ADDITIONAL_INFO failure.",
             "eStrKey": "Pattern"

src/sonic-yang-models/tests/yang_model_tests/tests_config/sonic-events-swss.json

@@ -113,6 +113,18 @@
             }
         }
     },
+    "SONIC_EVENTS_SWSS_PFC_STORM_VALID_WITH_ADDITIONAL_INFO_FRAGMENT": {
+        "sonic-events-swss:sonic-events-swss": {
+            "sonic-events-swss:pfc-storm": {
+                "ifname": "Ethernet0",
+                "queue_index": 0,
+                "queue_id": 0,
+                "port_id": 0,
+                "additional_info": "info1:1.2|info2:3.25",
+                "timestamp": "1985-04-12T23:20:50.52Z"
+            }
+        }
+    },
     "SONIC_EVENTS_SWSS_PFC_STORM_WITH_INVALID_ADDITIONAL_INFO_1": {
         "sonic-events-swss:sonic-events-swss": {
             "sonic-events-swss:pfc-storm": {

src/sonic-yang-models/yang-models/sonic-events-swss.yang

@@ -80,7 +80,7 @@ module sonic-events-swss {
 
             leaf additional_info {
                 type string {
-                    pattern '[-a-zA-Z0-9_]+:[-a-zA-Z0-9_]+(\|{1}[-a-zA-Z0-9_]+:[-a-zA-Z0-9_]+)*';
+                    pattern '[-a-zA-Z0-9_]+:[-a-zA-Z0-9_.]+(\|{1}[-a-zA-Z0-9_]+:[-a-zA-Z0-9_.]+)*';
                 }
                 description "Additional information to investigate PFC storm";
             }