Add SAI_MIRROR_SESSION_ATTR_POLICER to mirror session attributes (#899)

Enable the ability to attach a policer to the mirror session to throttle
the mirrored packets without affecting the original traffic.

Signed-off-by: Shu0T1an ChenG <shuche@microsoft.com>

inc/saimirror.h

@@ -290,6 +290,17 @@ typedef enum _sai_mirror_session_attr_t
      */
     SAI_MIRROR_SESSION_ATTR_GRE_PROTOCOL_TYPE,
 
+    /**
+     * @brief Mirror session policer object ID
+     *
+     * @type sai_object_id_t
+     * @flags CREATE_AND_SET
+     * @objects SAI_OBJECT_TYPE_POLICER
+     * @allownull true
+     * @default SAI_NULL_OBJECT_ID
+     */
+    SAI_MIRROR_SESSION_ATTR_POLICER,
+
     /**
      * @brief End of attributes
      */