[8.x] Build an on-demand filesystem disk

[browner12]

Occasionally I will need to access a file that does not currently reside within the root of a configured filesystem disk. I don't really want to create a full "disk" for them, because often this is a very one-off, or even temporary need. Sometimes it won't even get committed, but I just need it for a quick local dev use.

This PR allows us to make "on-demand" disks using the build() method.

There are 2 ways to build this on-demand disk. If you pass a string to the build() method, it will build a disk with a local driver, and the root pointed to the passed path.

Storage::build(storage_path('logs'));

You can also pass an array to have full control of the configuration:

Storage::build([
    'driver'     => 'local',
    'root'       => 'my-custom-path',
    'url'        => 'my-custom-url',
    'visibility' => 'public',
]);

[taylorotwell]

@browner12 did you have a big reason for storing the disk in the ondemand key? Should we just return it and not store it?

[taylorotwell]

Removed the part that cached it on the disks array - if that was super important let me know.

[browner12]

AFAIK not important. Only did it because the other ones did it.