AI issue description autocomplete

What does this MR do and why?

FE for https://gitlab.com/gitlab-org/gitlab/-/issues/406791+

Add a trigger + component to fill out issue description template using AI.

Screenshots or screen recordings

How to set up and validate locally

1. Set up an API key - https://gitlab.slack.com/archives/C0517A9PZ8S/p1681445186381439
2. Enable the feature flags

   Feature.enable(:openai_experimentation)
   Feature.enable(:generate_description_ai)   

3. Visit any issue page, or create a new issue. Project must be public
4. Click to edit the description
5. Select a template from the dropdown (if project has templates)
6. Click "Autocomplete"
7. Enter a description, submit form
8. Wait
9. Issue description updated

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.