AWS STS/Credential Support

[texastony]

User Story

As a Dafny developer,
I want to use AWS STS to retrieve credentials for specific roles,
rather than only relying on the environments default credential provider,
so that I may test how my Dafny source code handles
access denied exceptions or
other credential specific behavior.

Current State

The AWS Service clients supported by Smithy-Dafny can only use
the default credential provider.

The client construction behavior does not allow
Dafny developers to construct a service client using
a given set of credentials.

This is a regression from "native" development,
where at least some developers (cough-Crypto-Tools-cough)
regularly switch credentials to test or demonstrate features.

Some other thoughts

STS support is a start,
but there will need to be some additional changes to all
AWS SDK Client creation methods such that the clients
can be built with the given credentials.