This refactors it so "Defaults provider" is only responsible for "defaults"
Locale handling and management of locale settings is moved back into
SiteSettingExtension
This eliminates complex state management using DistributedCache and makes
it way easier to test SiteSettingExtension
This change-set allows setting different defaults for different locales.
It also:
- Adds extensive testing around site setting validation
- raises deprecation error if site setting has the default property based on env
- relocated site settings for dev and tests in the initializer
- deprecated client_setting in the site setting's loading process
- ensure it raises when a enum site setting being set
- default_locale is promoted to `required` category.
- fixes incorrect default setting and validation
- fixes ensure type check for site settings
- creates a benchmark for site setting
- sets reasonable defaults for Chinese
