Add S3Key implementation to construct S3 paths

[thekid]

Example:

use com\amazon\aws\S3Key;

-  $transfer= $s3->open('PUT', 'target/'.$file->filename, headers);
+  $transfer= $s3->open('PUT', new S3Key('target', $file->filename), $headers);

This supports arbitray filenames, including ones with characters that need to be encoded. The reason this class is called S3Key is that S3 does not double-encode the path component in the canonical request.

[thekid]

With PHP 8.0+, we could use string|S3Key here, see https://wiki.php.net/rfc/union_types_v2