allows you to define endpoints, like:

```rust
endpoint!(
    SetPin, // marker type
    PinSetting, // request type
    Result<(), PinError>,
    "pins/set",
);

// on the PC side
let res: Result<(), PinError> = client.send_resp::<SetPin>(&PinSetting { idx: 3, lvl: Level::High }).await?;
```