[5.4] Allow Slack Notifications to use image urls

[laurencei]

Currently the Laravel Notification channel for Slack seems to only support the 'icon_emoji'.

However Slack webhooks also allow you to provide an image URL - allowing you to use your own custom logo when sending a Slack notification - which is probably better for most Laravel Apps so they can include their logo?

This PR is fully BC. You can set a logo URL instead of an emoji:

public function toSlack($notifiable)
{
    return (new SlackMessage)
                ->from('My App')
                ->image('https://example.com/my-image.png')  
                ->content('This will be display my-image.png next to this alert on Slack');
}

p.s. I've tested this against the Slack API. If someone sets both an icon emoji and an icon image - Slack just uses the emoji with no complaints.