[8.x] Make FilesystemAdapter macroable

[harrygulliford]

This PR makes the \Illuminate\Filesystem\FilesystemAdapter class macroable. This class is used by the Storage facade.

A simple use-case I have encountered is wanting a clean way to return the root path of a particular storage disk.

Storage::macro('rootPath', function () {
    return $this->path('');
});

// ...

Storage::rootPath();

A slightly more complex use-case is a function to move files from one disk to another. This would be especially useful on Vapor, moving files from temporary storage (such as a dynamically generated spreadsheet or invoice) to a permanent s3 bucket (for a user to download).

Storage::macro('moveToDisk', function (string $path, string $disk = null) {
    Storage::disk($disk)->put($path, $this->get($path));
    $this->delete($path);
});

// ...

Spreadsheet::create(Storage::path('spreadsheet.xlsx'))
    ->addRow([ /* ... */ ]);

Storage::moveToDisk('spreadsheet.xlsx', 's3');