Add backport PR GHA (#1809)

* testing backport pr

* testing conditions run without merge

* more testing conditions

* trying app token

* test on branch

* add workflow dispatch

* switch to pull_request for testing

* cleaning up

* trigger workflow in 2 scenarios

1. pr labeled after merge, merge action
2. pr merged but label is already present

Author: swang392

.github/workflows/backport-pr.yml

@@ -0,0 +1,39 @@
+name: Backport PR
+on:
+  pull_request:
+    types: [ labeled, closed ]
+
+permissions: {}
+
+jobs:
+  backport:
+    name: Backport PR
+    runs-on: ubuntu-latest
+    if: >
+      github.event.pull_request.merged
+      && (
+        ( github.event.action == 'closed' && contains(github.event.label.name, 'backport') )
+        || 
+        ( github.event.action == 'labeled' && contains(github.event.label.name, 'backport') )
+      )
+    permissions:
+      contents: write
+      pull-requests: write
+    steps:
+      - uses: actions/create-github-app-token@21cfef2b496dd8ef5b904c159339626a10ad380e # v1.11.6
+        id: app-token
+        with:
+          app-id: ${{ vars.DD_GITHUB_TOKEN_GENERATOR_APP_ID }}
+          private-key: ${{ secrets.DD_GITHUB_TOKEN_GENERATOR_PRIVATE_KEY }}
+      - uses: tibdex/backport@9565281eda0731b1d20c4025c43339fb0a23812e # v2.0.4
+        with:
+          label_pattern: "^backport/(?<base>([^ ]+))$" 
+          labels_template: "<%= JSON.stringify([...labels, 'backport', 'bot']) %>"
+          github_token: ${{ steps.app-token.outputs.token }}
+          title_template: "[ Backport <%- base %> ] <%- title %>"
+          body_template: |
+            Backport <%- mergeCommitSha %> from #<%- number %>.
+
+            ___
+
+            <%- body %>