chore: clean up, refactor, reorganize, improve

• Add type safety
• Reduce duplicated/redundant code
• Improve spelling/grammar in public API names and internally, for instance
  • refreshDatsetMetadata() ==> refreshDatasetMetadata()
  • datasetsSize() ==> datasetSizes()
• Remove irrelevant GCS references, e.g.
  • getGcsUrl() ==> getStorageUrl()
• Refactor and reorganize code
• Remove unnecessary polymorphism