[Pipedv1] Plugin scaffold generator

What would you like to be added:

A CLI tool that generates the full scaffolding structure for a new PipeCD deployment plugin. The tool accepts flags for plugin name, stage definitions, and optional features (livestate, planpreview, rollback), then generates a ready-to-compile plugin with the correct directory layout, typed config structs, stage constants, pipeline builders, and per-stage executor stubs

Why is this needed: Creating a new plugin currently requires manually replicating boilerplate across multiple files (main.go, config/, deployment/pipeline.go, deployment/plugin.go, stage files, etc) while ensuring the correct interface is satisfied and naming conventions are followed

This is time consuming for developers. We need a way to reduces friction and a scaffold generator reduces

The inspiration is from how kubebuilder help developers build K8s API by generating boilerplate code, let them focus on the main logic. And I believe in our case a scaffold generator lets developers focus on implementing stage logic