Add GitLab VS Code extension to the tools-injector image

MR: fetch gitlab workflow and add to vscode-fork as... (gitlab-org/remote-development/gitlab-workspaces-tools!6 - merged)

Description

Customers can disable extensions marketplace or can bring their own extenisons marketplace which would not necessarily have the GitLab VS Code extension. Thus, we want to sideload this extension during workspace startup.

Add GitLab VS Code extension to the tools-injector image. Update the init_tools.sh script in the tools-injector image to configure the extensions as well.

Acceptance Criteria

GitLab VS Code extension is added to the tools-injector image, with all the configurations cortrectly applied.

• Add config to config.yaml.
• Update script scripts/build/fetch_assets.sh to get the URL of the asset to download.
• Update script scripts/build/prepare_docker_build.sh if necessary.
• Update Dockerfile
• Update scripts/copy_tools.sh and scripts/init_tools.sh
• Verify by following steps in https://gitlab.com/gitlab-org/remote-development/gitlab-remote-development-docs/-/blob/main/doc/tools-injector.md#testing-ci-built-images-before-publishing
• Pls note we would need to verify the tools-injector image is running fine with the gitlab vs code extension injected, and all existing extensions will still working fine.