EIP-4895 Withdrawals

Documentation for tests/shanghai/eip4895_withdrawals.

Generate fixtures for these test cases with:

fill -v tests/shanghai/eip4895_withdrawals

abstract: Tests EIP-4895: Beacon chain withdrawals

Test cases for [EIP-4895: Beacon chain push withdrawals as
operations](https://eips.ethereum.org/EIPS/eip-4895).