[5.5] Support custom URLs for AWS storage

[nesk]

AWS allows developers to customize their file URLs through DNS configuration. However, Laravel doesn't provide anything to generate customized file URLs for the s3 driver.

This pull-request fixes that by allowing users to define a url property in their configuration, just like the local driver:

's3' => [
    'driver' => 's3',
    'url' => 'https://files.example.org',
    'key' => env('AWS_KEY'),
    'secret' => env('AWS_SECRET'),
    'region' => env('AWS_REGION'),
    'bucket' => env('AWS_BUCKET'),
],

Storage::url('my/image.png'); // Returns: https://files.example.org/my/image.png

[tillkruss]

@nesk: This is great, but it's ignoring the root (path prefix) entirely.

's3' => [
    'driver' => 's3',
    'url' => 'https://files.example.org',
    'root' => 'footer',
],

This call:

Storage::url('my/image.png');

Returns:

https://files.example.org/my/image.png

But should return:

https://files.example.org/foobar/my/image.png

A simple fix is adding the root to the URL, but that's getting unreadable:

's3' => [
    'root' => env('S3_PREFIX', '_' . env('APP_ENV') . '/'),
    'url' => env('S3_URL', 'https://static.foxy.co') . env('S3_PREFIX', '_' . env('APP_ENV') . '/'),
]

What do you think about adding the root to the URL generation as well? That's the default behaviour when no url is set.