Make the `Block` parameter optional on the six state-reading methods,
defaulting to `latest` when omitted:
- `eth_getBalance`
- `eth_getCode`
- `eth_getStorageAt`
- `eth_getTransactionCount`
- `eth_getProof`
- `eth_getStorageValues`
This implements the behavior proposed in https://github.com/ethereum/execution-apis/pull/812.
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Co-authored-by: Sina M <1591639+s1na@users.noreply.github.com>