[9.x] Add conflict method to http client response

[lentex]

This PR adds a convenient method to check if a HTTP request resulted in a conflict (HTTP Status 409).

So instead of checking the status code manually, you can use this new method:

// Before
$response = Http::post('https://www.laravel.com', []);

if ($response->status() === 409) {
    doStuff();
}

// After
$response = Http::post('https://www.laravel.com', []);

if ($response->conflict()) {
    doStuff();
}

[taylorotwell]

It's really fine to just check the status.