arti-client: add ability to automatically bootstrap

The new BootstrapBehavior enum controls whether an unbootstrapped TorClient will bootstrap itself automatically (Ondemand) when an attempt is made to use it, or whether the user must perform bootstrapping themselves (Manual).

The new lazy-init example also shows how you could write a simple get_tor_client() function that used a global OnceCell to share a Tor client across an entire application, using the create_unbootstrapped API.

closes #278 (closed)