Settings: Support advanced setting registration

[joaomoreno]

We've had some cases in which extensions like Copilot Chat has the need for having advanced settings that aren't exposed to the user in the Settings UI or JSON editor. But over time we had to compensate for this and augment the JSON schema of the JSON editor to somewhat support this scenario.

Let's simply come up with an advanced settings concept. An advanced setting should be still registered with an advanced: true property. But:

• By default, users should not be able to discover advanced settings via the Settings UI or JSON editor.
• Unless they toggle View advanced settings (this can either be a setting or a global state value).
• (Optional) Insiders have View advanced settings set to true, by default.

cc @rzhao271