Fix/feat - `ContractOnGnosisChain` should have CHAIN_ID = 100

[gabrielfior]

Currently,

class ContractOnGnosisChain(ContractBaseClass):
    """
    Contract base class with Gnosis Chain configuration.
    """

    # This is defined like so because other chains (like Ethereum) rely on contracts that inherit
    # from ContractOnGnosisChain. To be re-evaluated on
    CHAIN_ID = RPCConfig().chain_id

@gabrielfior suggests that, on the above, CHAIN_ID=100 (for Gnosis).
@kongzii says OK, but this has implications, mostly on the SafeWatch Agent.

To be discussed how to best structure the contract classes.