Update sub_structs.rs to add missing users field in Trade struct

[r0b3r7137]

Hello

As per the doc at https://hyperliquid.gitbook.io/hyperliquid-docs/for-developers/api/websocket/subscriptions, the websocket trade response is as following

interface WsTrade {
  coin: string;
  side: string;
  px: string;
  sz: string;
  hash: string;
  time: number;
  // tid is 53-bit hash of (buyer_oid, seller_oid). 
  // For a globally unique trade id, use (block_time, coin, tid)
  tid: number;  
  users: [string, string] // [buyer, seller]
}

But the users field is missing from the struct in the SDK.