Add Typings, where appropriate, to backend packages [Feature Request]

```
from label_studio_ml.model import LabelStudioMLBase # type: ignore
from label_studio_ml.utils import ( # type: ignore
```

While the code I am creating, adding or modifying uses Type Hints (and checks), label_studio_ml packages are not Typed.
Generally, I try to reduce or eliminate un-typed code (i.e. 'any' variables). Adding a `types-label_studio_ml` package would be beneficial.
While adding Typings does take time and can seem tedious, it might help with reducing edge-case errors and overall improve the maintainability of the code.

Source: https://realpython.com/lessons/pros-and-cons-type-hints/