[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

Start by inspecting the existing plugin layout, including main.go, config/, deployment/pipeline.go, deployment/plugin.go, and the stage files, to identify the interfaces and naming conventions the generator must reproduce. Define the CLI flags for plugin name, stages, and optional features, then verify that generated output has the requested structure, typed config, stage constants, pipeline builders, executor stubs, and compiles.