Improve typings comparing to the original SDK version

[just-boris]

Describe the problem underlying the enhancement request

There is a known issue on AWS SDK v3 typings: https://github.com/aws/aws-sdk-js-v3/issues/1613

Marking everything as possibly undefined undermines the whole point of typings in the first place.

Since this package tries to reuse the upstream type definitions, it is suffering from the same issue too.

Describe the solution you'd like (if any)

The aws-lite project could provide enhanced typings for the most common use-cases

Describe alternative solutions you've considered

For now I am just using non-null assertion ! everywhere

Additional context or notes

This is not much of a feature request, more like a suggestion for to take an opportunity