feat(swagger): extends sm agent /node_info' with storage_size`.

This extends `/node_info` agent response with `storage_size` - total disk size
in bytes. It's needed for the backup manifest and should be calculated
on the agent side.

Refs: #4130

v3/swagger/agent.json

@@ -1687,6 +1687,10 @@
         "data_directory": {
           "description": "First entry from `data_file_directories` list from scylla config file.",
           "type": "string"
+        },
+        "storage_size": {
+          "description": "Disk size of the first entry from `data_file_directories` list from scylla config file. In bytes.",
+          "type": "integer"
         }
       }
     }