ci: add PR labeler workflow

Label based on target release branch. Labels must be manually defined to
avoid giving the action unnecessary permissions. Release labels must be
defined in the config after each release is forked.

Signed-off-by: George Sapkin <george@sapk.in>

.github/labeler.yml

@@ -0,0 +1,7 @@
+# branches
+"OpenWrt 23.05 (end of support)":
+- base-branch:
+  - "openwrt-23.05"
+"OpenWrt 24.10":
+- base-branch:
+  - "openwrt-24.10"

.github/workflows/labeler.yml

@@ -0,0 +1,20 @@
+name: 'Pull Request Labeler'
+on:
+  - pull_request_target
+
+permissions:
+  contents: read
+
+jobs:
+  labeler:
+    permissions:
+      contents: read
+      pull-requests: write
+
+    name: Pull Request Labeler
+    runs-on: ubuntu-slim
+    steps:
+      - uses: actions/labeler@v6
+        with:
+          repo-token: '${{ secrets.GITHUB_TOKEN }}'
+          sync-labels: true