Add autoload parameter to register_setting function

[pbearne]

This commit introduces an autoload parameter in the register_setting function, allowing users to specify if a setting should be autoloaded. Autoloaded settings are loaded on every page, improving the performance for settings that are frequently used. The change includes both the addition of the autoload parameter in the registration function as well as the corresponding changes in the database layout to store this information. This also includes a modification to wp_default_autoload_value filter to reflect the new autoload state. Changes are carried out across multiple files like theme-options.php, option.php, featured-content.php etc., for proper implementation.

Trac ticket: https://core.trac.wordpress.org/ticket/61522

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