Missing py.typed marker in box_sdk_gen causes mypy import-untyped error

When I run `mypy` on my project, it highlights the following import:

```python
from box_sdk_gen import ...
```

with the error:

```
error: Skipping analyzing "box_sdk_gen": module is installed, but missing library stubs or py.typed marker [import-untyped]
```

This can be simply resolved by adding a `py.typed` file to the root of the `box_sdk_gen` package to indicate that the package includes type hints.

[More details](https://typing.python.org/en/latest/spec/distributing.html#packaging-type-information).