feat: add gas fee strategy system with examples

- Add GasFeeStrategy module supporting normal and high-low fee strategies
- Initialize gas fee strategies in all examples
- Update documentation and README with gas fee strategy configuration
- Improve nonce cache documentation emphasizing latest nonce usage
- Adjust default gas fee configuration parameters
- Remove duplicate imports and unused constant references

examples/trading_client/src/main.rs

@@ -35,6 +35,8 @@ async fn create_solana_trade_client() -> AnyResult<SolanaTrade> {
     ];
     let trade_config = TradeConfig::new(rpc_url, swqos_configs, commitment);
     let solana_trade_client = SolanaTrade::new(Arc::new(payer), trade_config).await;
+    // init gas fee strategy
+    sol_trade_sdk::common::GasFeeStrategy::init_builtin_fee_strategies();
     println!("SolanaTrade client created successfully!");
     Ok(solana_trade_client)
 }