Improve RANDAO derivation logic in PureEngineClient

[coderabbitai]

Background

In PR evstack/go-execution-evm#30, we identified that the current implementation of derivePrevRandao in PureEngineClient simply returns the block height as a hash:

func (c *PureEngineClient) derivePrevRandao(blockHeight uint64) common.Hash {
    return common.BigToHash(new(big.Int).SetUint64(blockHeight))
}

Issue

While this implementation is feasible for test networks, production consensus protocols typically require actual random values. Using block height as a deterministic input for RANDAO doesn't provide the randomness properties expected in a production environment.

Potential Solutions

1. Implement a proper Verifiable Random Function (VRF)
2. Integrate with a secure random source
3. Implement a more sophisticated deterministic but unpredictable algorithm

References

• Original PR discussion: feat: restart repo with PureEngineClient go-execution-evm#30 (comment)