Get a block by hash in JSON format.

This structure extends the standard Block model to provide additional blockchain traversal capabilities by including a reference to the next block in the chain. This is particularly useful for blockchain explorers and analysis tools that need to navigate the chain in a forward direction.

The structure preserves all fields from the base Block model through embedding, while adding the NextBlock field that can be used for chain traversal. This approach maintains compatibility with existing Block-based code while extending functionality.

Fields: Block: Embedded Block model containing all standard block data including header, transactions, etc. NextBlock: Hash of the next block in the chain, or nil if this is the tip of the chain