Improve action output by using workflow step summaries

### TL;DR

Use GitHub [step summaries](https://github.blog/2022-05-09-supercharging-github-actions-with-job-summaries/) to improve readability and easier viewing of outputs from guardian when run in github actions.

### Detailed design

Using step summaries, add a step summary to show the terraform command outputs (plan, apply, etc) in a streamlined format so users can see information easier in the GitHub UI, rather than digging through the output logs.

### Alternatives considered

_No response_

### Additional information

_No response_

Find where guardian outputs terraform command results (plan, apply) in its GitHub Actions workflow. Look for the code that generates logs and integrate GitHub's step summary API to format these outputs. Check the repository for existing GitHub Actions workflows to understand the current output structure. Test by running a workflow locally or in a fork to see the new summary appear in the GitHub UI.

索引モデルが issue の本文から書いたものです。