contracts: Add rootClaim(uint256 l2ChainId) to IDisputeGame

[Inphi]

In preparation for interop, let's extend the IDisputeGame interface to support the rootClaim(uint256 l2ChainId) returns (Claim) method.

This new method will allow users to retrieve the Claim for a particular L2 chain ID. For pre-interop dispute games, this simply returns the root claim. But for the SuperDisputeGame.sol, this should return the claimed output root of the given L2 chain ID. For now, the SuperDisputeGame.sol implementation should revert until it can be created with a super root preimage.