Typing annotations

I've created typing annotations for `LambdaContext` and corresponding classes.

https://github.com/toidi/awslambdaric-stubs

It's useful in lambda handlers:

```python
import typing
if typing.TYPE_CHECKING: # pragma: no cover
from awslambdaric.lambda_context import LambdaContext

def lambda_handler(event: dict, context: "LambdaContext") -> typing.Any:
print(context.memory_limit_in_mb)
...
```

Perhaps, it would be better to move typing definition in this repository?