Add status column and state machine to workflows

What does this MR do and why?

This MR adds

• status column to duo_workflows_workflows table
• status state machine to Ai::DuoWorkflows::Workflow model

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

1. bundle exec rails db:migrate
2. Check that migration run successfully and the new column is created

Related to #469168 (closed)