Output in GitHub Actions format

[xnyo]

We want to run plugin-validator as part of plugin-ci-workflows (see grafana/plugin-ci-workflows#354).

It would be useful to output in the format used by GitHub Actions workflow commands using a config flag.

Examples in GHA docs:

• https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-commands#setting-a-warning-message
• https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-commands#setting-an-error-message

[xnyo]

Done in #455