Refactor repeated code in witness generation

In #5708, `generate_witness_from_account_updates` was introduced, and it has a significant amount of duplicated code with `generate_witness_for_blocks_with_fee_configs`. This approach was chosen for simplicity and to speed up the merge of the PR, since both functions are long and difficult to follow. We should extract the duplicated logic into a helper function and use it in both places.
