Introduce backport GHA (#8368)

Context
Introduces backport GitHub action for automation-aided propagation of changes between branches.

usage:
comment
/backport to <target bracnh>
on PR that needs to be backported (ideally after merging). New PR will be created backporting to the requested branch.

.github/workflows/backport.yml

@@ -0,0 +1,19 @@
+name: Backport PR to branch
+on:
+  issue_comment:
+    types: [created]
+
+permissions:
+  contents: write
+  issues: write
+  pull-requests: write
+  actions: write
+
+jobs:
+  backport:
+    uses: dotnet/arcade/.github/workflows/backport-base.yml@main
+    with:
+        pr_description_template: |
+          Backport of #%source_pr_number% to %target_branch%
+
+          /cc %cc_users%