add contracts.bytecode_hash field

[allenday]

Rationale for hashing the bytecode is that there are many addresses with identical code. We can find the unique set of contracts with less i/o if there's a hash field. This also allows for more efficient search for new contracts vs. all previous in combination with the date partition.

Can use FARM_FINGERPRINT for this:
https://cloud.google.com/bigquery/docs/reference/standard-sql/hash_functions#farm_fingerprint

Change needs to be here, etc:
https://github.com/blockchain-etl/ethereum-etl-airflow/blob/master/dags/resources/stages/raw/schemas/contracts.json#L9