github: Add backport label if pull request has Mergifyio backport comment

Useful to filter out which PRs should be backported yet.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>

.github/labeler.yml

@@ -0,0 +1,2 @@
+backport:
+  - '(Mergifyio backport )'

.github/workflows/labeler.yml

@@ -0,0 +1,19 @@
+name: Labeler
+
+on:
+  pull_request_target:
+    types:
+      - opened
+      - reopened
+
+jobs:
+  labeler:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: write
+    steps:
+    - uses: github/issue-labeler@v2.5
+      with:
+        repo-token: "${{ secrets.GITHUB_TOKEN }}"
+        configuration-path: .github/labeler.yml