Add SAI_MIRROR_SESSION_ATTR_LABEL to mirror session attributes (#2180)

Similar to the label used in SAI policer object, mirror session label attribute can be used to identify the same user mirror session configuration, e.g., in warmboot use case.

Signed-off-by: Running Huang <runningh@nvidia.com>

Author: running-h

inc/saimirror.h

@@ -394,6 +394,15 @@ typedef enum _sai_mirror_session_attr_t
      */
     SAI_MIRROR_SESSION_ATTR_GRE_HEADER_FIRST_16BIT,
 
+    /**
+     * @brief Label attribute used to uniquely identify mirror session.
+     *
+     * @type char
+     * @flags CREATE_AND_SET
+     * @default ""
+     */
+    SAI_MIRROR_SESSION_ATTR_LABEL,
+
     /**
      * @brief End of attributes
      */