Add automated labeler job

Rather than modifying the labels manually, which we often forget, we can
label the PRs automatically. This will make it easier to search PRs.

.github/labeler.yml

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+ci:
+  - changed-files:
+    - any-glob-to-any-file: .github/**
+test:
+  - changed-files:
+    - any-glob-to-any-file: fuzz/**
+    - any-glob-to-any-file: '*/tests/**'
+    - any-glob-to-any-file: 'dep_test'
+    - any-glob-to-any-file: 'contrib/test.sh'
+    - any-glob-to-any-file: '*/contrib/test.sh'
+doc:
+  - changed-files:
+    - any-glob-to-any-file: '**/*.md'

.github/workflows/manage-pr.yml

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+name: Manage PR
+on:
+  - pull_request_target
+
+jobs:
+  labeler:
+    permissions:
+      contents: read
+      pull-requests: write
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout master
+        uses: actions/checkout@v4
+        with:
+          path: master
+      - name: Checkout merge commit
+        uses: actions/checkout@v4
+        with:
+          path: merge
+          ref: "refs/pull/${{ github.event.number }}/merge"
+      - name: Generate label config
+        run: cd master && SCAN_DIR=../merge ./contrib/gen_label_config.sh
+      - name: Update labels
+        uses: actions/labeler@v5
+        with:
+          configuration-path: master/.github/labeler.yml

contrib/gen_label_config.sh

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+#!/bin/bash
+
+set -e
+
+# Generates the label configuration using crates in the repository.
+# The label configuration is appended to the labeler config file.
+
+config=.github/labeler.yml
+
+if [ -n "$SCAN_DIR" ];
+then
+	scan_dir="$SCAN_DIR"
+else
+	scan_dir=.
+fi
+
+if [ "$1" '!=' "--force" ] && ! git diff --exit-code "$config";
+then
+	echo "Error: $config is not committed."
+	echo "Refusing to overwrite it to prevent disaster."
+	echo "Run the script with --force to override this."
+	exit 1
+fi
+
+excluded_crates="fuzz|dep_test"
+
+CRATES="`cd "$scan_dir" && cargo metadata --no-deps --format-version 1 | jq -j -r '.packages | map(.manifest_path | rtrimstr("/Cargo.toml") | ltrimstr("'$PWD'/")) | join(" ")'`"
+
+for crate in $CRATES;
+do
+	if echo "$crate" | grep -qE "$excluded_crates";
+	then
+		continue
+	fi
+
+	echo "C-$crate:" >> "$config"
+	echo "  - changed-files:" >> "$config"
+	echo "    - any-glob-to-any-file: $crate/**" >> "$config"
+done