Create a one-to-many relationship coroutines function for drogon_models

I know one more feature that seems to still be missing in drogon_model.
Currently, Drogon already supports coroutines, but when using drogon_model to generate relationships, it still does not generate functions that can be used with co_await.

For example:
```cpp
auto a{co_await users.getA()};
```

I have to user callback approach, you can check in my project https://github.com/[bac-ptit/vip](https://github.com/bac-ptit/vip)