Process Withdrawals

[gakonst]

Describe the feature request

Post-Shanghai, withdrawals are being enabled. This has the following implications:

1. Types: Headers + Bodies now have a Withdrawals field, populated by the CL's Engine API calls (tracked in Engine API Changes for Shanghai #1297)
2. Execution: For every block, go over the withdrawals array and increase the balance of each account by the withdrawal amount

What needs to change:

1. Add withdrawals_root: Option<H256> to Header and other Header-like types
2. Add withdrawals: Vec<Withdrawal> (definition) to Block and other Block-like types
3. During header verification in Beacon Consensus, if we're in Shanghai, the withdrawals_root MUST be Some, and if we're not in Shanghai it MUST be None
4. After execution for a block, it should go over the array of withdrawals and directly update the balances of the accounts corresponding to each withdrawal. This means that withdrawal balances are applied at the END of the block after all txs for that block have been executed. Ref

Additional context

Geth's changes: ethereum/go-ethereum@2a2b041#diff-89272f61e115723833d498a0acbe59fa2286e3dc7276a676a7f7816f21e248b7