Introduce a separate data type for autocomplete interactions

The interaction payload for autocompletes is slightly different. It's
missing the `resolved` data map and reports only for the current
autocompleted option value the `focused` field and its value as a string.

The focused option value is modeled as an enum variant of `CommandDataOptionValue`.

```
enum CommandDataOptionValue {
    Autocomplete { name: String, kind: CommandOptionType, value: String },
}
```

The focused option value can be retrieved via `AutocompleteData::focused_option()`
from the `AutocompleteData::options` vector.