Indexes the new validator withdrawal objects (from Shanghai/Capella fork: https://eips.ethereum.org/EIPS/eip-4895)
- new table `eth.withdrawal_cids`
- new column `withdrawals_root` in `eth.header_cids`
- add unit tests
- use new external stack repo in CI/CT job (cerc-io/fixturenet-eth-stacks#14)
Reviewed-on: #25
