Add allowedExtensions security setting

[google-labs-jules]

Added a new security setting allowedExtensions which takes a list of regex patterns. If configured, extensions can only be installed or loaded if their source matches one of the patterns. This setting overrides blockGitExtensions (i.e., if an extension is explicitly allowed, it can be installed even if git extensions are blocked globally).

---

PR created automatically by Jules for task 5057098429212851851 started by @kevinjwang1

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.