Add an `is_finished` method to the `StorageClient` interface

[janbuchar]

• Reasoning here
  • refactor!: Align RequestQueueClient interface with its Python counterpart crawlee#3729 (comment)
  • refactor!: Align RequestQueueClient interface with its Python counterpart crawlee#3729 (comment)
• This can be made non-breaking by making the method optional and assuming is_empty <=> is_finished if is_finished is not provided