Introduces the ability to sign generated URLs for enhanced security and access control.
This feature is activated by configuring a `PrivateKey`, `UID`, and `ValidDuration` in the driver settings. If a private key is provided, the driver will sign the output URLs, making them time-limited based on the `ValidDuration`. The `ValidDuration` defaults to 30 minutes if not specified.
The core signing logic is encapsulated in the new `sign.go` file. The `driver.go` file integrates this signing process before returning the final URL.
