Allow internal-executor thread to be disabled on a per-Connection basis

This could be a part of the ConnectionBuilder, where you either pass in an existing Arc<Executor> or just set a bool that commits you to getting the executor out of the built Connection and ticking it.

Reason: library crates that depend on zbus might not realize they need to disable default features if they aren't creating the Connection, or they might need the feature enabled if the library actually creates Connection objects in some cases. However, this shouldn't force all users to launch threads.

Once implemented, this could either remove the internal-executor feature, or just make it toggle the default value of the option in new builders.