[CI] Add support for post merge testing

Author: shahmishal

.github/workflows/post_merge.yml

@@ -0,0 +1,16 @@
+name: Post merge
+
+on:
+  push:
+    branches: [main]
+
+jobs:
+  tests:
+    name: Test
+    uses: swiftlang/github-workflows/.github/workflows/swift_package_test.yml@main
+  soundness:
+    name: Soundness
+    uses: swiftlang/github-workflows/.github/workflows/soundness.yml@main
+    with:
+      license_header_check_enabled: false
+      license_header_check_project_name: "Swift.org"