[5.3] Add fluent builder for SlackMessageAttachmentField

[freekmurze]

Currently Laravel only has a fields method to attach some fields to a SlackAttachment

The example from the documentation:

 return (new SlackMessage)
                ->success()
                ->content('One of your invoices has been paid!')
                ->attachment(function ($attachment) use ($url) {
                    $attachment->title('Invoice 1322', $url)
                               ->fields([
                                    'Title' => 'Server Expenses',
                                    'Amount' => '$1,234',
                                    'Via' => 'American Express',
                                    'Was Overdue' => ':-1:',
                                ]);
                });

That's nice, but under the hood the short property of the fields is being set to false. For longer text in attachment there's no way to set this to true. So, an attachment like this (taken from the Slack docs) isn't possible.

This PR adds a builder for a SlackMessageAttachmentField in the same spirit as SlackMessageAttachment. Using this builder the short property can be set to true by calling the doNotDisplaySideBySide-method.

Here's how the builder can be used:

//in a Notification

public function toSlack($notifiable)
    {
        return (new SlackMessage)
            ->content('Content')
            ->attachment(function ($attachment) {
                $attachment
                    ->title('Laravel', 'https://laravel.com')
                    ->content('Attachment Content')

                    //build up an attachment field
                    ->field(function($attachmentField) {
                        $attachmentField
                            ->title('Special powers')
                            ->content('Zonda')
                            ->dontDisplaySideBySide();
                    });

                    //manually add one
                    ->field('Project', 'Laravel')
            });
    }

Hope you'll like it!