ci(automation): added openGate build spec (#6108)

## Problem
We close source gate during the start of a release to prevent unwanted
commits coming in, but we have to remember to manually open it again.

We want to automatically re-open the source gate after a release is
triggered.

This PR creates the buildSpec file to be used by codeBuild project in
our release pipeline to open the source transition.

Related infra CR:
https://code.amazon.com/reviews/CR-162245234/revisions/1#/diff

---

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License: I confirm that my contribution is made under the terms of the
Apache 2.0 license.

Author: tomcat323

buildspec/release/35opengate.yml

@@ -0,0 +1,18 @@
+version: 0.2
+
+phases:
+    install:
+        runtime-versions:
+            nodejs: 16
+
+    pre_build:
+        commands:
+            - STAGE_NAME=SourceWithGit
+            - PIPELINE=$(echo $CODEBUILD_INITIATOR | sed -e 's/codepipeline\///')
+    build:
+        commands:
+            - |
+                aws codepipeline enable-stage-transition \
+                  --pipeline-name "$PIPELINE" \
+                  --stage-name "$STAGE_NAME" \
+                  --transition-type "Inbound"