[2.x] Allow to override Nexmo client via NexmoMessage

[tzurbaev]

This PR adds ability to override underlying Nexmo\Client instance with custom client. This helps in situations when you have some customer/DB-stored credentials and you need to send SMS using given credentials (not with the system-wide that are stored in services.php).

Example code:

<?php

declare(strict_types=1);

namespace App\Notifications;

use Illuminate\Notifications\Messages\NexmoMessage;
use Illuminate\Notifications\Notification;
use App\Models\Company;
use Nexmo\Client;

class ExampleNotification extends Notification
{
    private Company $company;

    public function __construct(Company $company)
    {
        $this->company = $company;
    }

    public function toNexmo($notifiable): NexmoMessage
    {
        return (new NexmoMessage('Something happened!'))
            ->from($this->company->nexmo_from)
            ->via($this->getCompanyClient());
    }

    private function getCompanyClient(): Client
    {
        return new Client(
            new Client\Credentials\Basic(
                $this->company->nexmo_key,
                $this->company->nexmo_secret
            )
        );
    }
}