Improve GNMI_CLIENT_CERT table to support multiple roles. (#21849)

Improve GNMI_CLIENT_CERT table to support multiple roles.

Why I did it
GNMI need support multiple roles.
HLD: sonic-net/SONiC#1967

Work item tracking
Microsoft ADO (number only):31561802
How I did it
Change GNMI_CLIENT_CERT table yang model.

How to verify it
Pass all test case.

This PR depends on following PRs:
sonic-net/sonic-gnmi#366
sonic-net/sonic-mgmt#17866

Author: liuh-80

src/sonic-yang-models/tests/files/sample_config_db.json

@@ -1414,10 +1414,14 @@
         },
         "GNMI_CLIENT_CERT": {
             "testcert1": {
-                "role": "RW"
+                "role": [
+                        "RW"
+                ]
             },
             "testcert2": {
-                "role": "RO"
+                "role": [
+                        "RO"
+                ]
             }
         },
         "TUNNEL": {

src/sonic-yang-models/tests/yang_model_tests/tests/gnmi.json

@@ -20,7 +20,7 @@
     },
     "GNMI_CLIENT_CERT_LIST_TABLE_WITH_MISSING_ROLE": {
         "desc": "CLIENT_CERT_LIST_TABLE_WITH_MISSING_ROLE failure.",
-        "eStrKey": "Mandatory"
+        "eStrKey": "MinElements"
     },
     "GNMI_CLIENT_CERT_LIST_TABLE_WITH_VALID_CONFIG": {
         "desc": "TABLE WITH VALID CONFIG."

src/sonic-yang-models/tests/yang_model_tests/tests_config/gnmi.json

@@ -108,11 +108,15 @@
                 "GNMI_CLIENT_CERT_LIST": [
                 {
                     "cert_cname": "testcert1",
-                    "role": "RW"
+                    "role": [
+                        "RW"
+                    ]
                 },
                 {
                     "cert_cname": "testcert2",
-                    "role": "RO"
+                    "role": [
+                        "RO"
+                    ]
                 }
                 ]
             }

src/sonic-yang-models/yang-models/sonic-gnmi.yang

@@ -107,9 +107,9 @@ module sonic-gnmi {
                         "client cert common name";
                 }
 
-                leaf role {
+                leaf-list role {
                     type string;
-                    mandatory true;
+                    min-elements 1;
                     description
                         "role of client cert common name";
                 }