feat: add shoes-tester CLI tool for testing shoes providers

Add shoes-tester, a standalone CLI tool that allows developers to test
shoes provider implementations without running the full myshoes server.
This tool supports both AddInstance and DeleteInstance operations with
two modes: simple mode (with custom setup script) and script generation
mode (automatically generates GitHub runner setup scripts).

Changes:
- Add cmd/shoes-tester/main.go with CLI implementation
- Export GetSetupScript in pkg/starter for use by shoes-tester
- Update protobuf generated files with newer protoc version
- Add comprehensive documentation in docs/03_how-to-develop-shoes.md

The tool supports various options including resource types, labels,
GitHub App authentication, and JSON output format for automation.

Author: whywaita