fix: serialise Buffer as hex string

[ns212]

This pull request introduces new endpoints for block information and improves serialisation of byte Buffer.

BigInt and JSON Handling Improvements:

• Removed the json-bigint dependency and its usage throughout the codebase, simplifying JSON serialization logic. [1] [2] [3]
• Added a global toJSON method to the BigInt prototype, ensuring that bigint values are serialized as strings in JSON outputs.
• Configured Express with a custom JSON replacer to convert Buffer objects to hexadecimal strings during JSON serialization.

New API Endpoints for Block Data:

• Added /tip/height endpoint to retrieve the current tip block height, using cache when available and falling back to a live query if necessary.
• Added /:height/header endpoint to fetch block header information for a specific height, including the block hash and height.