fix(engine): expose session close

[DecFox]

Fixes ooni/probe#2810

Proposed Changes

• Exposes the close function for the engine session
• Invokes the close function while downloading urls for webconnectivity to ensure the running psiphon tunnel (and engine session) is closed before the experiment run
• Changes in engine api surface: refactor(oonimkall): expose the session close call probe-cli#1657

[aanorbel]

@DecFox , we need to update the engine version to ensure the build works before we can merge this.