Give GenericSchemes a name, default proposal params

Feature request: Let GenericScheme instances be configured with a name, default ABI and contract address. dApps such as Alchemy could display the name to automatically identify each GenericScheme, and supply `null`s for the ABI and contract address when creating a proposal, to cause `execute` to use the defaults.

These configurations would be part of the scheme parameters, settable by SchemeRegistrar.

dApps would obtain the default ABI from the GenericScheme params and parse it to identify any contract.function params that do not have values. For those params that do not have values, the dApp would generate a form enabling the user to enter the values. The dApp would poke these values into the ABI and submit that completed ABI to `proposeCall` (when proposals are created).

These features would enable dApps like Alchemy to use GenesisScheme instances without requiring any custom code to support the GenericScheme.