Data plane verification (#4639)

* Add signature to blob

* Change Signable trait to support returning references to signable data

* Add signing to broadcast

* Verify signatures in window_service

* Add testing for signatures to erasure

* Add RPC for getting current slot, consume RPC call in test_repairman_catchup for more deterministic results

client/src/mock_rpc_client_request.rs

@@ -60,6 +60,7 @@ impl GenericRpcClientRequest for MockRpcClientRequest {
                 serde_json::to_value(response).unwrap()
             }
             RpcRequest::GetTransactionCount => Value::Number(Number::from(1234)),
+            RpcRequest::GetSlot => Value::Number(Number::from(0)),
             RpcRequest::SendTransaction => Value::String(SIGNATURE.to_string()),
             _ => Value::Null,
         };