- introduce ConfigBuilder struct for constructing ClientConfig instances
- add methods to ConfigBuilder for setting various configuration parameters
- implement a build method to create the ClientConfig instance
- add a default implementation for ConfigBuilder
- enhance ClientConfig with persist_path for token persistence
- update ClientConfig::new_from_configuration to use config_root.full_path()
- add ClientConfig::persist_path() to report the full path to the file to persist tokens
- adds `ClientConfig` struct for managing Gmail client settings
- supports loading credentials from environment variables or a JSON file
- provides methods to access the application secret and configuration root
