Interop: op-supervisor: Implement backend CheckBlock

[ajsutton]

optimism/op-supervisor/supervisor/backend/backend.go

Lines 90 to 93 in cc86dc1

	func (su *SupervisorBackend) CheckBlock(chainID *hexutil.U256, blockHash common.Hash, blockNumber hexutil.Uint64) (types.SafetyLevel, error) {
	// TODO(protocol-quest#288): hook up to logdb lookup
	return types.CrossUnsafe, nil
	}

[protolambda]

Completed in #11586