Extract CI test commands to shell scripts and refactor TLS features

[yoshidan]

Summary

• Extract CI test commands to shell scripts under .github/scripts/
• Refactor TLS feature flags across crates

CI Script Extraction

Background

Since we use pull_request_target, workflow files are loaded from the base branch (main).
This means changes to feature flags in fork PRs are not reflected in CI, causing test failures.

Changes

• Add .github/scripts/ directory
• Extract each test command to individual shell scripts
• Update ci.yaml to call scripts instead of inline commands

Contributors can now modify scripts in their fork and have those changes reflected in CI.
Security is maintained through the existing safe to test label requirement.

TLS Feature Refactoring

Changes

• Rename default-tls to rustls-tls as the default TLS backend
• Add native-tls feature to support native TLS backend
• Apply to: foundation/auth, pubsub, spanner, storage, kms, bigquery, artifact-registry