Adopt utility process for shared process and set `app.enableSandbox()`

This is a follow up from https://github.com/microsoft/vscode/issues/92164 and covers remaining work to eventually enable sandboxed renderers fully in Electron via app.enableSandbox().

This means that our shared process has to move away from a node.js enabled browser window to the new utility process.

Breaking down the usages today:

• extension management
• settings sync
• profiles
• terminals
• file watcher

Some initial thoughts:

• the shared process should probably just change to be a utility process as a first step
  • however, any code that relies on the browser window network stack instead has to leverage Electrons net APIs from the electron-main process to not loose proxy support
  • this can probably be done by implementing some kind of IRequestService that is backed by a main process service implementation
• any child process has to decide whether it wants to lift up to a utility process off the main process or remain inside the shared process

//cc @alexdima

[deepak1556]

Message port is limited to operating only from the utility process. I would prefer to keep it this way since otherwise we could have untrusted child processes talking directly to the renderer and serves as a way to break out of the renderer sandbox.

[sandy081]

settings sync is safe to be removed as it does not use any non sandbox stuff like node apis.