[12.x] Added JsonSerializable interface to Uri Class

[devajmeireles]

Relates to #56094

---

This PR aims to ensure the Uri class is converted to JSON, fixing the issue: #56094

Before this PR, an API that uses the url attribute as using the new AsUri cast, will respond like this:

Route::get('/testing', function () {
    return User::first()->only('id', 'url');
});

Will result in it:

{
    "id": 1,
    "url": {}
}

After this PR, the value will be correctly converted to the expected Uri output:

{
    "id": 1,
    "url": "https://sanford.biz/..."
}

[devajmeireles]

I don't think the test failure is related exactly to the changes in this PR.