test: add the loadtime tool (Backport #9342) (#9358)

* 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.

* lint

test/loadtime/README.md

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+# loadtime
+
+This directory contains `loadtime`, a tool for generating transaction load against Tendermint.
+`loadtime` generates transactions that contain the timestamp corresponding to when they were generated
+as well as additional metadata to track the variables used when generating the load.
+
+
+## Building loadtime
+
+The `Makefile` contains a target for building the `loadtime` tool.
+
+The following command will build the tool and place the resulting binary in `./build/loadtime`.
+
+```bash
+make build
+```
+
+## Use
+
+`loadtime` leverages the [tm-load-test](https://github.com/informalsystems/tm-load-test)
+framework. As a result, all flags and options specified on the `tm-load-test` apply to
+`loadtime`.
+
+Below is a basic invocation for generating load against a Tendermint websocket running
+on `localhost:25567`
+
+```bash
+loadtime \
+    -c 1 -T 10 -r 1000 -s 1024 \
+    --broadcast-tx-method sync \
+    --endpoints ws://localhost:26657/websocket
+```