Enable VS Code private extensions marketplace for Web IDE in workspaces

Issue: Enable VS Code private extensions marketplace f... (#451941 - closed)

What does this MR do and why?

Enable VS Code private extensions marketplace.

• See #451419

See also related issue for these settings in the non-remote-dev Web IDE: Create Instance-Level and Root-Group-Level Exte... (#451871)

MR acceptance checklist

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How to set up and validate locally

1. Feature.enable(:allow_extensions_marketplace_in_workspace)

2. Start a local workspace and verify the default openvsx-org marketplace is used (select an extension and hover over the Marketplace link).

3. Override the necessary env var to pick a different marketplace as documented in #451419. I.e., override to the official marketplace.visualstudio.com:

export GITLAB_REMOTE_DEVELOPMENT_VSCODE_EXTENSIONS_GALLERY='{"service_url":"https://marketplace.visualstudio.com/_apis/public/gallery","item_url":"https://marketplace.visualstudio.com/items","resource_url_template":"https://{publisher}.vscode-unpkg.net/{publisher}/{name}/{version}/{path}"}'

4. Start a NEW local workspace and see that the specified marketplace is used in the Web IDE (select an extension and hover over the Marketplace link) and that it allows installation of extensions.

5. Feature.disable(:allow_extensions_marketplace_in_workspace)

6. Start a NEW local workspace and see that the marketplace is disabled.

Screenshots

With ENV var set

Marketplace is enabled and overridden, extensions can be successfully installed.

Without ENV var set

Marketplace is enabled and uses default openvsx marketplace.

With feature flag disabled

Marketplace is disabled