feat: add postprocess_cmd option to run commands after code generation

[amithkk]

This PR adds a new postprocess_cmd option to plugin configurations in buf.gen.yaml. This allows users to specify commands that run after code generation for each plugin, enabling automatic formatting, linting, or other post-processing of generated files.

Motivation

Solves #3207

Many protoc plugins generate code that doesn't conform to a project's formatting standards and some languages need postprocessing (eg. python) for the modules to be importable. Currently, users must manually run formatters and cleanup scripts after buf generate, or maintain separate scripts. This feature allows integrating post-processing directly into the generation workflow.

Common use cases:

• Running gofmt or goimports on generated Go code
• Running proletariat on generated python code to fix imports

Usage

version: v2
plugins:
  - local: protoc-gen-python_betterproto
    out: gen/python
    postprocess_cmd:
      - "ruff check --fix $out"
      - "black $out"

Supported Variables

Commands support variable substitution:

• $out - output directory (resolved with any base output path)
• $name - plugin name
• $opt - plugin options (comma-separated)
• $path - plugin path (for local plugins)
• $strategy - generation strategy ("directory" or "all")

Unit tests have been added

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