ci(automation): added openGate build spec

[tomcat323]

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.

[hayemaxi]

Can we add this to the existing file 30closegate.yml

[hayemaxi]

• This pull request implements a feat or fix, so it must include a changelog entry (unless the fix is for an unreleased feature). Review the changelog guidelines.

  • Note: beta or "experiment" features that have active users should announce fixes in the changelog.
  • If this is not a feature or fix, use an appropriate type from the title guidelines. For example, telemetry-only changes should use the telemetry type.

The PR title/commit should use ci instead of feat

[tomcat323]

Can we add this to the existing file 30closegate.yml

I don't think so, because some actions only uses closegate but not opengate.