Make governance parameters for the Ethereum bridge contract addresses

[james-chf]

Currently we hardcode dummy Ethereum addresses for a couple of the bridge smart contracts

namada/apps/src/lib/node/ledger/ethereum_node/oracle.rs

Lines 21 to 23 in 67d9d43

	/// Dummy addresses for smart contracts
	const MINT_CONTRACT: EthAddress = EthAddress([0; 20]);
	const GOVERNANCE_CONTRACT: EthAddress = EthAddress([1; 20]);

We should instead have a governance parameter for the Ethereum address of the "proxy" smart contract and use that for getting the "bridge" and "governance" smart contract addresses. This should be initialized to the zero Ethereum address at genesis as the Ethereum bridge should not launch straight away at chain launch. The Ethereum oracle component should switch away from using the hardcoded dummy addresses.

We should also have a parameter for the wnam ERC20 contract address.

[james-chf]

Will start being used in #686