Add action to copy metadata to backport PRs

This adds a GitHub Action that triggers whenever Mergify opens a PR
against one on the stable branches, and copies across the relevant
labels and milestone from the base PR (if any).  This just ensures that
things like the changelog tag and the tracking milestone are reliably
tracked without a maintainer needing to do it manually.

.github/workflows/backport.yml

@@ -0,0 +1,50 @@
+name: Backport metadata
+
+# Mergify manages the opening of the backport PR, this workflow is just to extend its behaviour to
+# do useful things like copying across the tagged labels and milestone from the base PR.
+
+on:
+  pull_request:
+    types:
+      - opened
+    branches:
+      - 'stable/*'
+
+jobs:
+  copy_metadata:
+    name: Copy metadata from base PR
+    runs-on: ubuntu-latest
+    if: github.repository == 'Qiskit/qiskit' && github.actor == 'mergify[bot]'
+
+    permissions:
+      pull-requests: write
+
+    steps:
+      - name: Copy metadata
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          set -e
+          # Split Mergify's ref name (e.g. 'mergify/bp/stable/0.25/pr-10828') on the final '-' to
+          # get the number of the PR that triggered Mergify.
+          IFS='-' read -ra split <<< "${{ github.event.pull_request.head.ref }}"
+          base_pr=${split[-1]}
+
+          gh pr --repo "${{ github.repository }}" view "$base_pr" --json labels,milestone > base_pr.json
+
+          add_labels="$(jq --raw-output '[.labels[].name] - ["stable backport potential"] | join(",")' base_pr.json)"
+          echo "New labels: '$add_labels'"
+
+          milestone="$(jq --raw-output '.milestone.title // ""' base_pr.json )"
+          echo "Milestone: '$milestone'"
+
+          echo "Targetting current PR '${{ github.event.number }}'"
+          # The GitHub API is sometimes weird about empty values - the REST API certainly treats
+          # "add-label" with an empty input not as a no-op but as a clear of all labels.
+          if [[ -n "$add_labels" && -n "$milestone" ]]; then
+            gh pr --repo "${{ github.repository }}" edit "${{ github.event.number }}" --add-label "$add_labels" --milestone "$milestone"
+          elif [[ -n "$add_labels" ]]; then
+            gh pr --repo "${{ github.repository }}" edit "${{ github.event.number }}" --add-label "$add_labels"
+          elif [[ -n "$milestone" ]]; then
+            gh pr --repo "${{ github.repository }}" edit "${{ github.event.number }}" --milestone "$milestone"
+          fi