[5.3] Setup additional configuration for Slack Client.

[srmklive]

As described in #15837, currently there is no way we can specify additional options for Guzzle like headers or proxy when using Slack notifications. This PR allows you to specify such settings.

Whenever you create a notification through php artisan make:notification and add toSlackmethod. Then you can specify additional parameters for Guzzle like this:

$options = [
    'proxy' => [
        'http'  => 'tcp://localhost:8125',
        'https' => 'tcp://localhost:9124',
    ]
];
return (new SlackMessage)
        ->options($options)
        ->content('Some Tests');