Improve GNMI_CLIENT_CERT table to support multiple roles.

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

#### Which release branch to backport (provide reason below if selected)

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- [x] 202006
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- [ ] 201811
- [ ] 201911
- [ ] 202006
- [ ] 202012
- [ ] 202106
- [ ] 202111
- [ ] 202205
- [ ] 202211
- [ ] 202305

#### Tested branch (Please provide the tested image version)

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- Please provide tested image version
- e.g.
- [x] 20201231.100
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- [ ] <!-- image version 1 -->
- [ ] <!-- image version 2 -->

#### Description for the changelog
Improve GNMI_CLIENT_CERT table to support multiple roles.

<!--
 Ensure to add label/tag for the feature raised. example - PR#2174 under sonic-utilities repo. where, Generic Config and Update feature has been labelled as GCU.
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#### Link to config_db schema for YANG module changes
<!--
Provide a link to config_db schema for the table for which YANG model
is defined
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#### A picture of a cute animal (not mandatory but encouraged)

Author: mssonicbld

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

@@ -1390,10 +1390,14 @@
         },
         "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";
                 }