Implement Proof-of-Work (PoW) algorithm

[devduart]

Implement a basic Proof-of-Work mechanism to simulate mining. Each block must compute a valid hash with a certain number of leading zeros.

Tasks

• Add a Nonce field to the Block struct.
• Implement a MineBlock(difficulty int) function that updates the hash until the difficulty is satisfied.
• Integrate mining into the NewBlock function.
• Make difficulty configurable via a global constant or config.
• Write unit tests to validate mining output.

Reference

• https://en.bitcoin.it/wiki/Proof_of_work