Add fzf support to bin/feature-flag

What does this MR do and why?

This MR is a follow-up to Introduce new feature flag types, constraints a... (!136599 - merged), which adds fzf support to the bin/feature-flag script.

You can see this change in action at https://asciinema.org/a/YeSnpXkXbUimivLj1cGLWjJ8S

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. Install fzf if it's not installed

   brew install fzf

2. Execute the script

   ./bin/feature-flag foo

gitlab-org/quality/engineering-productivity/team#352 (closed)