Simplify API of create_unbootstrapped, maybe

It looks like create_unbootstrapped just got a third argument:

    pub fn create_unbootstrapped(
        runtime: R,
        config: TorClientConfig,
        autobootstrap: BootstrapBehavior,

I'm very glad to have this feature, but I have a question: Are we confident that autobootstrap is the very last setup option that we'll want to add here? Otherwise, I'm worried that we'll want to keep adding more arguments in the future.

Three ways I can think of to solve this:

1. Declare that yes, autobootrap is special and we won't want to add any more.

2. Replace autobootstrap with a ClientSetupOptions value that contains autobootstrap, and can later be extended to include more options.

3. Remove autobootstrap, and add a separate function to toggle it: fn bootstrap_on_demand(&self, autobootstrap: bool).

@eta, @Diziet what do you think here?

Since this would be an API issue I'd like to get it solved before 0.1.0