Kintsugi v2: implement RANDOM

[garyschulte]

Description

Besu added random to the blockHeader in amphora, but disabled it due to it being out of scope. For the kintsugi v2 spec, we fully implement EIP-4399:

• use repurpose/replace mixHash with random post-TTD
  • in the block hash
  • in the RLP encoding (save on 32 bytes)
  • random should be persisted in the db
    • we should drop the unused blockheader fields from the db-serialization post-TTD, but this is an optimization
• change to the EVM opcode 0x44(DIFFICULTY):
  • rename to RANDOM
  • return the value of random rather than difficulty(which will be fixed at 0)

Acceptance Criteria

• implemented in merge-branch
• passing merge-fuzz for v2
• passing evm reference tests