Simplify inline module data handling.

Feature Description

As suggested by @aaemnnosttv on Slack, we should follow up on the work started in #10988 and simplify the implementation.

Do not alter or remove anything below. The following sections will be managed by moderators only.

Acceptance criteria

• The Module_With_Inline_Data_Trait trait should be removed, while keeping the Module_With_Inline_Data interface.
• The Modules::inline_modules_data() method should be updated to iterate over modules that implement the interface and collect their data.
• Modules should return unkeyed data arrays from get_inline_data, with slug-based keying handled by the Modules class.

Implementation Brief

• Remove the Module_With_Inline_Data_Trait trait from the following modules:
  • Ads.php
  • Analytics_4.php
  • Sign_In_With_Google.php
• Update the get_inline_data() method in those classes to return the inline data directly, without keying it by the module slug.

For example, change this:

return array(
    self::MODULE_SLUG => $data,
);

to this:

return $data;

• Update the inline_modules_data method in Modules.php to populate the inline data from modules that implement the Module_With_Inline_Data interface. Each module's inline data should be added to $modules_data, using the module's slug ($module::MODULE_SLUG) as the key and the data returned by $module->get_inline_data() as the value.
• Delete the Module_With_Inline_Data_Trait.php file.

Test Coverage

• Update the inline data related tests of the affected modules (Analytics_4Test.php, AdsTest.php, Sign_In_With_GoogleTest.php) to use the get_inline_data method directly instead of relying on the googlesitekit_inline_modules_data hook.
• Add new tests to ModulesTest.php to cover the inline_modules_data method and verify that it correctly adds the inline_module_data of classes that implement the Module_With_Inline_Data interface

QA Brief

Note: I copied these steps from #10988, as the main check here is also just to ensure none of the data went missing.

• Set up Site Kit with Analytics and Ads enabled.
• Go to the Site Kit page.
• Type _googlesitekitModulesData in the console.
  • The object should have an ads key, whose value is an object containing:
    • supportedConversionEvents: an array of supported conversion events.
      • As a sanity check, install WooCommerce. This array should contain WooCommerce-related events (add-to-cart, purchase).
  • The object should have an analytics-4 key, whose value is an object containing:
    • customDimensionsDataAvailable
    • isWebDataStreamUnavailable
    • resourceAvailabilityDates
    • tagIDMismatch
    • newEvents
    • lostEvents
    • newBadgeEvents
  • Without setting up Sign In With Google, the object should not have sign-in-with-google.
  • After setting up Sign In With Google, the object should have the sign-in-with-google key, whose value is an object containing:
    • isWooCommerceActive
    • isWooCommerceRegistrationEnabled

Changelog entry

• Simplify inline module data handling.