Add functionality to retrieve one specific dataset

### StatGPT Backend version

0.1.0

### What is the problem this feature will solve?

Current implementations like `get_dataset_by_source_id` or `list_available_dataset` loads all channel datasets which can be slow when we want to retrieve only one specific dataset.

### What is the proposed feature or solution?

Add `get_dataset_by_urn` method which will retrieve one specific dataset by urn specified

### What alternatives have you considered?

None alternatives, as all methods use implementation which loads all datasets