Add endpoint for time to beat

Hey,

thanks for this easy to use Swift-wrapper for the IGDB API.

It seems that there is no function for the ["time to beat" endpoint](https://api-docs.igdb.com/?swift#game-time-to-beat). The model (`Proto_GameTimeToBeat`) seems to be there, but I don't see a function to query the endpoint.

Would you mind adding it?