Add release_pipeline.yml workflow

[YixuanWang-99]

Description

Add release_pipeline.yml workflow with release_approval, build_and_test_maxtext_package, and build_release_candidate_images jobs

Tests

Workflow is tested through workflow_dispatch: https://github.com/AI-Hypercomputer/maxtext/actions/runs/26662269195

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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