Add sample_txs function to perf_utils shared crate (#4104)

Shared code between bench-tps and bench-exchange

client/src/lib.rs

@@ -1,6 +1,7 @@
 pub mod client_error;
 mod generic_rpc_client_request;
 pub mod mock_rpc_client_request;
+pub mod perf_utils;
 pub mod rpc_client;
 pub mod rpc_client_request;
 pub mod rpc_request;