Add `getInputBox` function to `ICartesiDApp`

[guidanoli]

📚 Context

There should be an entry point for outsiders to know which input box is the DApp listening to. This may be particularly useful for consensuses.

✔️ Solution

Add a getInputBox function that returns the address of the input box where inputs to the DApp should be added to.

📈 Subtasks

• [ ] Add function to ICartesiDApp and CartesiDApp
• [ ] Add parameter to CartesiDApp constructor
• [ ] Add parameter to functions and events from ICartesiDAppFactory and CartesiDApp
• [ ] Add unit tests

[guidanoli]

FYI @tuler