[6.x] Allow absolute file path for Storage::putFileAs()

[SjorsO]

Currently, trying to store an absolute file path to storage is inconvenient. As far as I know the easiest way to do it is like this:

Storage::put(
    '/some/path/foo.txt',
    $handle = fopen($filePath, 'r')
);

fclose($handle);

With this PR, you can do the following instead:

Storage::putFileAs('/some/path/', $filePath, 'foo.txt');

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I haven't changed Storage::putFile. That method uses the UploadedFile to guess the file extension and generate a random file name. See PR #31040