test: add the loadtime tool (#9342)

This pull request adds the loadtime tool. This tool leverages the tm-load-test framework. Using the framework means that the only real logic that needs to be written is the logic for Tx generation. The framework does the rest.

The tool writes a set of metadata into the transaction, including the current transaction rate, number of connections, specified size of the transaction, and the current time.

test/loadtime/basic.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+set -euo pipefail
+
+# A basic invocation of the loadtime tool.
+
+./build/loadtime \
+    -c 1 -T 10 -r 1000 -s 1024 \
+    --broadcast-tx-method sync \
+    --endpoints ws://localhost:26657/websocket
+