ClientContext should be a type, not String

[ktoso]

Expected behavior

The ClientContext is often of well known shape:

It should handle the typing more gracefully I think, see docs:
https://docs.aws.amazon.com/lambda/latest/dg/nodejs-context.html

See also: #167 (comment) for discussion on this

Actual behavior

It's a String today:

            let clientContext: String?

[sebsto]

Still valid for v2