Create centralised backport workflow (#17)

To support `backport` being used in multiple GitHub Actions:
- hazelcast/hazelcast-mono#3642
- hazelcast/hz-docs#1396

A composite GitHub Action has been created to centralise it's usage, to
make callers simpler.

Implementation is a sliced from `hazelcast-mono`, with some parameters
now explicit inputs.

Tested via [dummy PR](JackPGreen2/hz-docs#14) in
[test repo](https://github.com/JackPGreen2/hz-docs) that [backported
successfully](https://github.com/JackPGreen2/hz-docs/actions/runs/12029584772/job/33536129458)
(via JackPGreen2/hz-docs#26).

.github/actions/backport/action.yml

@@ -0,0 +1,83 @@
+name: Backport
+
+# A composite action that allows Backporting via GitHub Actions (e.g. to be driven based on labels etc)
+
+inputs:
+  GITHUB_TOKEN:
+    required: true
+  TARGET_BRANCH:
+    description: The branch in the repository to backport changes into
+    required: true
+  REF_TO_BACKPORT:
+    description: The reference of the commit to be backported
+    required: true
+
+env:
+  # Not possible to set this as a default
+  # https://github.com/orgs/community/discussions/46670
+  shell: bash
+
+runs:
+  using: composite
+  steps:
+    - name: Assert branch exists
+      id: assert-branch-exists
+      shell: ${{ env.shell }}
+      run: |
+        if git ls-remote --exit-code --heads origin "${{ inputs.TARGET_BRANCH }}"; then
+          echo "::debug::Branch ${{ inputs.TARGET_BRANCH }} exists"
+        else
+          echo "::error::Branch ${{ inputs.TARGET_BRANCH }} does not exist"
+          gh pr comment "${{ github.event.pull_request.number }}" \
+            --repo ${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY} \
+            --body "❌ The backport branch \`${{ inputs.TARGET_BRANCH }}\` doesn't exist."
+          echo "failure-already-reported=true" >> ${GITHUB_OUTPUT}
+          exit 1
+        fi
+      env:
+        GH_TOKEN: ${{ github.token }}
+
+    - name: Backport
+      shell: ${{ env.shell }}
+      run: |
+        # Git metadata is required but not available out-of-the-box, inherit from the action
+        git config user.name "${GITHUB_ACTOR}"
+        git config user.email "${GITHUB_ACTOR}@users.noreply.github.com"
+
+        # Add "upstream" remote as checkout action doesn't include by default
+        git remote add upstream "${{ github.event.repository.clone_url }}"
+        git fetch --all
+
+        backport_target_branch=upstream/"${{ inputs.TARGET_BRANCH }}"
+        echo "::debug::Running backport script to backport "${{ inputs.REF_TO_BACKPORT }}" into \"${backport_target_branch}\""
+
+        ${GITHUB_ACTION_PATH}/../../../backport \
+          "${{ inputs.REF_TO_BACKPORT }}" \
+          "${backport_target_branch}" \
+          --non-interactive
+      env:
+        GH_TOKEN: ${{ github.token }}
+
+    - name: Report errors
+      shell: ${{ env.shell }}
+      if: failure() && steps.assert-branch-exists.outputs.failure-already-reported != 'true'
+      run: |
+        echo ":error::Error running action"
+        echo "::group::Troubleshooting Information:"
+        echo "- Repositories' GitHub action configuration - ${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}/settings/actions"
+        echo "- Action does not run when triggered from a forked repo's PR?"
+        echo "    Enable \"Run workflows from fork pull requests\""
+        echo "- Backport fails with \"GraphQL: GitHub Actions is not permitted to create or approve pull requests (createPullRequest)\"?"
+        echo "    Either:"
+        echo "        * Enable \"Allow GitHub Actions to create and approve pull requests\""
+        echo "        * Use a different \"GITHUB_TOKEN\" with appropriate permissions"
+        echo "- \"GraphQL: Resource not accessible by integration (addComment)\"?"
+        echo "    Either:"
+        echo "        * Enable \"Send write tokens to workflows from fork pull requests\""
+        echo "        * Use a different \"GITHUB_TOKEN\" with appropriate permissions"
+        echo "::endgroup::"
+        gh pr comment "${{ github.event.pull_request.number }}" \
+          --repo ${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY} \
+          --body "❌ [Failed to backport](${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}/actions/runs/${GITHUB_RUN_ID}), change must be manually backported."
+      env:
+        GH_TOKEN: ${{ github.token }}