[dependencies] add Renovate config (#4236)

* Add configuration for automatically creating dependencies, and for pinning GitHub Actions digests
* Add a post-upgrade script that runs cargo-hakari.

Depends on oxidecomputer/renovate-config#5.

See [RFD 434](https://rfd.shared.oxide.computer/rfd/0434) and #4166.

.github/renovate.json

@@ -0,0 +1,9 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "local>oxidecomputer/renovate-config",
+    "local>oxidecomputer/renovate-config//rust/autocreate",
+    "local>oxidecomputer/renovate-config:post-upgrade",
+    "helpers:pinGitHubActionDigests"
+  ]
+}

tools/renovate-post-upgrade.sh

@@ -0,0 +1,42 @@
+#!/bin/bash
+
+# This script is run after Renovate upgrades dependencies or lock files.
+
+set -euo pipefail
+
+# Function to retry a command up to 3 times.
+function retry_command {
+    local retries=3
+    local delay=5
+    local count=0
+    until "$@"; do
+        exit_code=$?
+        count=$((count+1))
+        if [ $count -lt $retries ]; then
+            echo "Command failed with exit code $exit_code. Retrying in $delay seconds..."
+            sleep $delay
+        else
+            echo "Command failed with exit code $exit_code after $count attempts."
+            return $exit_code
+        fi
+    done
+}
+
+# Download and install cargo-hakari if it is not already installed.
+if ! command -v cargo-hakari &> /dev/null; then
+    # Need cargo-binstall to install cargo-hakari.
+    if ! command -v cargo-binstall &> /dev/null; then
+        # Fetch cargo binstall.
+        echo "Installing cargo-binstall..."
+        curl --retry 3 -L --proto '=https' --tlsv1.2 -sSfO https://raw.githubusercontent.com/cargo-bins/cargo-binstall/main/install-from-binstall-release.sh
+        retry_command bash install-from-binstall-release.sh
+    fi
+
+    # Install cargo-hakari.
+    echo "Installing cargo-hakari..."
+    retry_command cargo binstall cargo-hakari --no-confirm
+fi
+
+# Run cargo hakari to regenerate the workspace-hack file.
+echo "Running cargo-hakari..."
+cargo hakari generate