Stub vscode extension marketplace UI

Issue: Create skeleton for Extensions Marketplace sett... (#508977 - closed)

What does this MR do and why?

This MR sets up the UI for VSCode Extension Marketplace in application settings. For now, the settings are just editable through a simple textarea to support iteration of follow-up MR's. This is behind a feature flag disabled by default vscode_extension_marketplace_settings.

Screenshots or screen recordings

vscode_extension_marketplace_stub_ui_demo

UPDATE (2025-03-07):

Based on this thread, I opted to improve the error reporting here and use the api/v4 endpoint instead of the controller. The resulting error message looks like:

How to set up and validate locally

With `vscode_extension_marketplace_settings` feature flag enabled

1. Start GDK and visit /rails/features.
2. Search for vscode_extension_marketplace_settings (either in the page or with the search box) and activate the toggle.
3. Visit /admin/application_settings/general and search for VS Code Extension Marketplace near the bottom. NOTE: Sometimes feature flags are cached a bit heavily on the GDK. Try refreshing a couple of times if it doesn't show up right away.
4. Enter {} for Settings and click Save. EXPECT: No error should occur.
5. Enter {"enabled":true,"preset":"open_vsx"} for Settings and click Save. EXPECT: No error should occur.
6. Refresh the page and check that the settings were persisted.
7. Enter {"enabled":true,"preset":"bogus"} for Settings and click Save. EXPECT: An error should be displayed.

With `vscode_extension_marketplace_settings` feature flag disabled

1. Start GDK and visit /rails/features.
2. Search for vscode_extension_marketplace_settings (either in the page or with the search box) and deactivate the toggle.
3. Visit /admin/application_settings/general. EXPECT: No section should be rendered for VS Code Extension Marketplace.

Related to #508977 (closed)