Add CLI tool for downloading CockroachDB binaries with cross-platform support

[rail]

• Create cmd/download-crdb-binaries CLI tool with flags:

  • -platform (linux, darwin, windows) - defaults to runtime.GOOS
  • -arch (amd64, arm64) - defaults to runtime.GOARCH
  • -version (specific version or 'unstable') - defaults to 'unstable'
  • -output (custom directory) - defaults to temp directory

• Extend testserver package with platform-specific functions:

• Fix unstable binary filename generation:

  • Changed from 'cockroach-' to descriptive 'cockroach-latest'

• Add macOS ARM64 unsigned binary support:

  • Correct naming pattern: 'darwin-arm64.unsigned' for unstable builds
  • Enables downloading bleeding-edge builds on Apple Silicon

• Maintain backward compatibility:

  • Original functions delegate to new platform-specific versions
  • Existing test server functionality unchanged

• Remove unsupported musl detection.

The CLI tool now supports downloading any CockroachDB version for any supported platform/architecture combination, with 'unstable' as the sensible default for getting the latest bleeding-edge builds.

[cockroach-teamcity]

This change is