Provide a way for a contract developer to explain typed data in natural language

The same as #1842, but for [typed data](https://github.com/aeternity/aepp-sdk-js/blob/develop/docs/guides/typed-data.md)

```
  entrypoint explain(td: typedData): string =
    let operation = String.concat("Perform a ", td.operation)
    let parameter = String.concat(" operation with parameter ", Int.to_str(td.parameter))
    String.concat(operation, parameter)
```