Enable VS Code private extensions marketplace for Web IDE in workspaces via ENV vars

MR: Enable VS Code private extensions marketplace f... (!147718 - merged)

Description

This is the initial MVP for Investigate and break down tasks on how to conf... (#451419 - closed)

It allows the Web IDE in a workspace to configure the VS Code private extensions marketplace via instance-level ENV vars.

Acceptance Criteria

• Extension marketplace settings can be overridden by instance-level ENV vars.
• Add documentation on how to use new ENV vars (in standard place for ENV vars in our docs, unless there's a better place) (will be covered by Add docs for VS Code Marketplace settings vars (#452549 - closed))

Technical Requirements

See #451419 (closed) for more detailed technical requirements.

Design Requirements

This is a minimal implementation that only uses ENV vars, no design.