Add support for the 'metadata' config option

[diezit]

Support for this was added in the main package Superbalist/flysystem-google-cloud-storage, but was not added to this laravel integration yet. This PR fixes that.

The commit that added metadata support to the other repo is Superbalist/flysystem-google-cloud-storage@fedbdca

[felipemeddeiros]

Could you help me in how to implente a metadata configuration?

[ffflabs]

@felipemeddeiros you can do that already, for example:

     $disk = Storage::disk('gcs');
     $disk->put($filePath, $fileContents, [
            'metadata'=>[
                'cacheControl'=> 'public,max-age=86400',
                'contentType'=> 'image/jpeg'
            ]
     ]);

This PR would allow for a default metadata config applied to every upload which doesn't set an explicit config parameter

'gcs' => [
    'driver' => 'gcs',
    'project_id' => env('GOOGLE_CLOUD_PROJECT_ID', 'your-project-id'),
    ... other keys ...
     'metadata'=>[
           'cacheControl'=> 'public,max-age=86400',
     ]
]

usual metadata entries are described here: https://firebase.google.com/docs/storage/web/file-metadata