Typing for `BaseLambdaEvent.context` should be `LambdaContext` instead of dict

The `context` attribute does not match the implementation and docs.

https://github.com/aws/chalice/blob/a7a96df5809bc37a2174687f55760cdedcdc3ed6/chalice/app.pyi#L486-L490

Should be `chalice.local.LambdaContext` instead of `dict`.

Example docs:
https://aws.github.io/chalice/api.html?highlight=snsevent#SQSEvent.context

Start at chalice/app.pyi lines 486-490 and compare the annotation with chalice.local.LambdaContext and the linked API documentation. Confirm the existing implementation and docs use LambdaContext, then update the type declaration so BaseLambdaEvent.context matches them.

Written by the indexing model from the issue text.