Provide a config schema for the Book Visibility condition plugin

Problem/Motivation

The module is missing the config schema definition.

Steps to reproduce

Install the config_inspector → module and inspect the config schema when the plugin is used.

Proposed resolution

Use a more consistent configuration schema to ensure the config exports are consistent.

e.g. declaring a ConditionInterface::defaultConfiguration() method with a default value for book_visibility (so it's aware when the condition is not in use), and ensuring the data is normalized as a sequence rather than a key value map, so that we no longer need to call array_values() within the code.