Develop the execute-call Function with Static Dispatch

[CyrusVorwald]

• In the xCall implementation contract, define a function execute-call that processes incoming messages.
• Since dynamic dispatch is not possible, require that the target contract is known and registered in the xCall contract.
• Implement a registration mechanism where dApps can register their contract principal with xCall. This might have to be hard coding the principal in execute-call, or storing messages for the dApp to retrieve later instead of calling the dApp's handle-call-message.
• In execute-call, use the registered contract principal to call the dApp's handle-call-message function using contract-call?