ExternalDisputeID discovery (cross-chain)

[jaybuidl]

Follow-up on #1316

Add the HomeGateway.CrossChainDisputeIncoming event info to the subgraph Dispute entity in KlerosCore.ts.

    /// @dev To be emitted when a dispute is received from the IForeignGateway.
    /// @param _arbitrator The arbitrator of the contract.
    /// @param _arbitrableChainId The chain identifier where the Arbitrable contract is deployed.
    /// @param _arbitrable The address of the Arbitrable contract.
    /// @param _arbitrableDisputeID The identifier of the dispute in the Arbitrable contract.
    /// @param _arbitratorDisputeID The identifier of the dispute in the Arbitrator contract.
    /// @param _externalDisputeID An identifier created outside Kleros by the protocol requesting arbitration.
    /// @param _templateId The identifier of the dispute template. Should not be used with _templateUri.
    /// @param _templateUri IPFS path to the dispute template starting with '/ipfs/'. Should not be used with _templateId.
    event CrossChainDisputeIncoming(
        IArbitratorV2 _arbitrator,
        uint256 _arbitrableChainId,
        address indexed _arbitrable,
        uint256 indexed _arbitrableDisputeID,
        uint256 indexed _arbitratorDisputeID,
        uint256 _externalDisputeID,
        uint256 _templateId,
        string _templateUri
    );

https://github.com/kleros/kleros-v2/blob/d4f554be7f4c14eeb0d34b8ca514c09d84df715e/contracts/src/gateway/interfaces/IHomeGateway.sol#L25