Add output-file parameter to GitHub Action

[brijeshp56]

Description:
It would be helpful if the trufflehog@main GitHub Action supported
saving scan results directly to a file.

Proposed Solution:
Add an optional output-file input parameter to action.yml that
allows users to specify a file path for saving JSON results.

Example usage:

• uses: trufflesecurity/trufflehog@main
  with:
  output-file: scan-results.json
  extra_args: --results=verified,unknown

Current Workaround:
Users must run TruffleHog via Docker with output redirection instead
of using the action.

Benefits:

• Easier artifact upload in CI/CD pipelines
• Consistent with action-based workflows
• Better integration with other GitHub Actions
• Users can parse the output file and post findings as comments on pull
  requests, which cannot be achieved without using Docker or installing
  TruffleHog directly on the executor
• Enables custom post-processing workflows (filtering, formatting,
  notifications) without requiring Docker runtime or CLI installation

Thanks,
Brijesh