Optional notification channel packages.

[dkulyk]

• Laravel Version: 5.7.16
• PHP Version: 5.3
• Database Driver & Version: MariaDB 10.2

Description:

Not all need to install nexmo or slack notifications

Steps To Reproduce:

$ composer show | grep notification-channel
laravel/nexmo-notification-channel    v1.0.1             Nexmo Notification Channel for laravel.
laravel/slack-notification-channel    v1.0.3             Slack Notification Channel for laravel.

[CrixuAMG]

@dkulyk if I understood correctly, they will be removed for Laravel 5.8. These have been imported into the composer.json to make sure no breaking change is introduced for this version.

[staudenmeir]

See #26689 (comment).