Bring Vulnerability Explanation to Beta on GitLab Duo Self Hosted

What does this MR do and why?

For #524917 (closed)

Bring Vulnerability Explanation feature to Beta on GitLab Duo Self Hosted

Feature selection page

Feature button

Feature in action

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Setup a runner in your GDK as per GDK documentation - https://gitlab.com/gitlab-org/gitlab-development-kit/blob/main/doc/howto/runner.md#create-and-register-a-local-runner
2. Create a new empty project on GDK
3. Copy over/recreate the files from https://gitlab.com/gitlab-examples/security/security-reports to be able to create a SAST vulnerability report with ease.
4. Once you're done copying the files, run a new pipeline on main. This should create vulnerability reports on the project.
5. Go to a SAST vulnerability within the project
6. Click on Explain with AI button or type /vulnerability_explain directly on the Duo Chat window.

MR acceptance checklist

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

Related to #524917 (closed)