This commit introduces the test-gcr tool, a Go application designed to automate testing local sidekick changes against the google-cloud-rust repository.

The tool includes the following features:

• Command-line flag parsing for configuring repository paths, branches, pass-through arguments for sidekick and cargo.
• Pre-flight checks for dependencies (Go, Rustc), path validity,
  Git repository status, and remote configuration.
• Orchestration of fetching updates, resetting the google-cloud-rust
  repository, running sidekick refresh, formatting, and executing tests.
• Granular dry-run mode (--dry-run)
• Comprehensive unit tests for validation and command execution logic.
• README.md with usage instructions and examples.