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!

首先重现导入 aws_encryption_sdk 时报告的 mypy 错误，然后检查该包的公共类型接口以及 issue 中提到的不兼容问题。确定该包是否可以使用 py.typed 标记，或者是否需要 stubs；当 mypy 能够在没有这些错误的情况下分析受支持的公共导入时，即可认为工作完成。

由索引模型根据 Issue 内容生成。