[2.x] Add initial props resolver for Initial Page Load

[michaelnabil230]

This PR adds support for an initial prop option in Inertia.js shared props. It allows developers to define props that should be sent only with the initial page load and excluded from subsequent Inertia responses.

This enhancement is particularly useful for values like configuration settings, application metadata, or other static data that doesn't need to be included with every request — reducing payload size and improving response efficiency.

Example:

public function initialPropsResolver(): callable
{
    return function (Request $request) {
        return [
            'locale' => 'en',
            'appName' => 'Test App',
            'translations' => [
                'en' => 'English',
                'es' => 'Spanish',
            ],
        ];
    };
}

In this example, the defined props (locale, appName, translations) will be shared only on the first Inertia response.

Why this matters:

As discussed in Inertia.js Discussion #993, there's a recurring need to send certain props — such as app settings, tokens, or translation strings — only with the initial page load, during the app's initial load. This feature provides a clean, built-in solution for that use case.

Note

If this PR proves useful and is approved for integration, I'd be happy to work on adding the corresponding support in the Inertia.js client-side adapter as well.

See: #760