senders and receivers are empty in the response of dense_pull_ledger_diff

[Dengjianping]

When runtime(wasm) gets the data from related storages, then pass it to client(host, jsonrpsee), codec(skip) will skip to encode both two fields.
The response like

{
  should_continue: false,
  receivers: '',
  senders: '',
  senders_receivers_total: '0x02000000000000000000000000000000',
  next_checkpoint: null
}

This is caused by the macro codec(skip)

ub struct DensePullResponse {
    ...
    #[codec(skip)]
    pub receivers: String,

    #[codec(skip)]
    pub senders: String,
    ...
}

But runtime(codec) doesn't allow to use String, removing #[codec(skip)] will report compilation error.

#[derive(Clone, Debug, Encode, Default, Eq, Hash, Decode, PartialEq, TypeInfo)]
      |                        ^^^^^^ the trait `WrapperTypeEncode` is not implemented for `String`