[sonic-yang-models] add ACL flex counter group (#8909)

Why I did it
To add ACL FC to the YANG model.

How I did it
Added to the YANG model and added a UT.

How to verify it
Together with depends PRs. Run ACL/Everflow test suite.

Signed-off-by: Stepan Blyshchak <stepanb@nvidia.com>

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

@@ -31,6 +31,9 @@
                 },
                 "QUEUE_WATERMARK": {
                     "FLEX_COUNTER_STATUS": "enable"
+                },
+                "ACL": {
+                    "FLEX_COUNTER_STATUS": "enable"
                 }
             }
         }

src/sonic-yang-models/yang-models/sonic-flex_counter.yang

@@ -151,6 +151,16 @@ module sonic-flex_counter {
                 }
             }
 
+            container ACL {
+                /* ACL_STAT_COUNTER_FLEX_COUNTER_GROUP */
+                leaf FLEX_COUNTER_STATUS {
+                    type flex_status;
+                }
+                leaf FLEX_COUNTER_DELAY_STATUS {
+                    type flex_delay_status;
+                }
+            }
+
         }
         /* end of container FLEX_COUNTER_TABLE */
     }