Provide new site settings loader that uses manage display, etc rather than flattening

Problem/Motivation

The flattening approach works fine for simple label fields like credits, etc, but it has caused problems breaking from standard Drupal architecture as this module gets more usage.

Proposed resolution

Would suggest instead we have a loader that renders site settings entities in a standard way, respecting manage display.

It then cannot inject the site settings to every template, so keeping with the goal of the module to add labels very simply to eg footer, header, other regions where site settings are useful, suggest we add a Twig Filter and make that the recommended way. Similar to what Twig Tweak offers but without the dependency on that module perhaps, or it could be a dependency of that loader?

Remaining tasks

1. Provide annotation for Loader; id + label + whether to autoload or not
2. Provide new loader with autoload disabled
3. Provide update hook to maintain status quo for loader
4. Let site settings config form choose loader
5. Only auto-load site settings into templates if autoload is enabled on the loader
6. Remove calls to site_settings.loader service, replace with plugin manager load definition
7. Move SiteSettingsLoaderBase functionality into FlattenedSiteSettingsLoader
8. Move SiteSettingsRenderer into FlattenedSiteSettingsLoader
9. Change /admin/content/site-settings to a View
10. Update hook to install the View maintaining roughly the status quo for the listing
11. Add 'Create setting' to View header for not yet created site settings
12. Add 'Add another' to operations for site settings that allow multiple
13. Build FullSiteSettingsLoader
14. Add tests
15. Update documentation

Follow-up issues

1. https://www.drupal.org/project/site_settings/issues/3398584
2. https://www.drupal.org/project/site_settings/issues/3399190

User interface changes

Site settings config form allows choice of loader

Data model changes

Multiple site settings loaders will be available + developers can optionally override/provide their own.