Feature request: Add support for typing with mypy

### Problem:

Using the aws_encryption_sdk in a project with mypy produces this error:

```
error: Skipping analyzing "aws_encryption_sdk": module is installed, but missing library stubs or py.typed marker
```

### Solution:

I attempted to simply add the py.typed marker file to my virtualenv and it seems not as simple, a lot of other incompatibilities.
Maybe stubs or .pyi could be provided

Thank you very much!