fix issue with block number loading

[malteish]

my canister failed to load the current block number because the Block type contained some fields that caused trouble:

• difficulty -> was contained in response, but raised error anyway
• totalDifficulty -> was not contained in response
• baseFeePerGas -> was contained in response, but raised error anyway

Marking them optional resolved the issue.

The errors looked like this:
[116870. 2024-10-27T06:07:41.440086079Z]: [TRAP]: Panicked at 'Call failed (update_last_observed_block_number): (CanisterError, "failed to decode canister response as (evm_rpc_canister_types::MultiGetBlockByNumberResult,): Fail to decode argument 0")', canisters/chain_fusion/src/logs.rs:172:10