Add Arm64 CI support for Unit tests in Vitess

[ranimandepudi]

This PR adds native Arm64 support to Vitess’s unit test GitHub Actions workflows. The goal is to ensure Vitess tests run cleanly on modern Arm-based environments alongside existing coverage.

Changes made:
• Added matrix.arch: [amd64, arm64] to all unit test workflows
• Dynamic runs-on logic to use the correct GitHub-hosted runner
• Parameterized etcd download using ${ARCH} to ensure correct binary per platform
• Safely skipped MySQL 5.7 workflows on Arm64, since Oracle does not provide .deb packages for Arm
• Updated test/templates/unit_test.tpl to preserve changes with make generate_ci_workflows

Unit test workflows now Arm64 ready are:
unit_test_mysql80.yml
• unit_test_mysql84.yml
• unit_test_evalengine_mysql80.yml
• unit_test_evalengine_mysql84.yml
• unit_race.yml
• unit_race_evalengine.yml
Excluded:
• unit_test_mysql57.yml
• unit_test_evalengine_mysql57.yml

Further Work that is in progress:

• Add Arm64 support for e2e tests
• Enable multi-arch docker builds
• Documentation Updates

Thanks for reviewing this PR and for all your work on Vitess!
This change adds Arm64 support to the unit test CI workflows, enabling testing across both amd64 and arm64 platforms using GitHub Actions.
I’m happy to refine or break this PR into smaller pieces if that helps.

[vitess-bot]

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[deepthi]

Looks like DCO is failing.
If the dev env is setup by following the instructions on the website https://vitess.io/docs/contributing/build-on-macos/, the source dev.env step actually sets up the necessary git pre-commit hooks that will prompt you to signoff each commit.