(feat) Extract some logic to a userInstance method.

[lucasmichot]

This PR extracts some of the logic of the \Laravel\Socialite\Two\AbstractProvider::user() method into a new \Laravel\Socialite\Two\AbstractProvider::userInstance() method.

Nothing changes, but each provider can now override userInstance() and had some extra logic or retrieve additional data.

Probably one of the first use case could be for the GitHub provider and its new expiring tokens: the token response now comes with a new field called refresh_token_expires_in (possibly null), that will can now be attached to the user in a simple way:

// in src/Two/GithubProvider.php
/**
 * {@inheritdoc}
 */
function userInstance(array $response, array $user)
{
    $this->user = parent::userInstance($response, $user);
    $this->user->refreshTokenExpiresIn = $response['refresh_token_expires_in'] ?? null;

    return $this->user;
}

---

Note:
I am not opinionated concerning the name of this method, that could be renamed.