Add a script to stress-test faucets (#4396)

## Motivation

On Testnet Babbage, we experienced a faucet failure (rather, DoS) when
many people tried to create wallets and chains at the same time. To make
sure it doesn't happen again, we'd like to stress-test the faucet with
artificial data.

## Proposal

Add a small bash script that executes `linera wallet init; linera wallet
request-chain` against a faucet many times in parallel, printing timing
information on stdout in JSON for analysis.

## Test Plan

Run against a local faucet.

## Release Plan

- Nothing to do / These changes follow the usual release cycle.

## Links

- [reviewer
checklist](https://github.com/linera-io/linera-protocol/blob/main/CONTRIBUTING.md#reviewer-checklist)

Author: Twey

scripts/faucet-stress-test.bash

@@ -0,0 +1,49 @@
+#!/usr/bin/env bash
+
+faucet_url=${1-http://localhost:8080}; shift
+count=${1-100}; shift
+wallet_base_path=${1-$(mktemp -dtu linera-faucet-stress-test.XXXXXX)/wallet}; shift
+
+wallets=()
+
+function cleanup {
+    rm -rf "${wallets[@]}"
+    rmdir --ignore-fail-on-non-empty "$wallet_base_path" 2>/dev/null
+}
+
+trap cleanup EXIT
+
+function create_wallet_and_claim_chain {
+    local path=$1; shift
+    wallets+=("$path")
+    mkdir -p $path
+    TIMEFORMAT='{ "operation": "init", "duration": %R, "wallet": "'"$path"'" }'
+    time {
+        LINERA_WALLET=$path/wallet.json \
+        LINERA_KEYSTORE=$path/keystore.json \
+        LINERA_STORAGE=rocksdb:$path/storage.db \
+        >/dev/null \
+        2>&3 \
+        linera wallet init --faucet $faucet_url
+    }
+    TIMEFORMAT='{ "operation": "request-chain", "duration": %R, "wallet": "'"$path"'" }'
+    time {
+        LINERA_WALLET=$path/wallet.json \
+        LINERA_KEYSTORE=$path/keystore.json \
+        LINERA_STORAGE=rocksdb:$path/storage.db \
+        >/dev/null \
+        2>&3 \
+        linera wallet request-chain --faucet $faucet_url
+    }
+}
+
+TIMEFORMAT='{ "operation": "stress-test", "duration": %R }'
+{
+    time {
+        for i in $(seq $count)
+        do
+            create_wallet_and_claim_chain $wallet_base_path/$i &
+        done
+        wait
+    }
+} 3>&2 2>&1