Add way for `AWSShellScript` to use other shells besides bash

**Is your feature request related to a problem? Please describe.**

Users have been using the AWS Tools for Powershell task just to get AWS credentials in a Powershell environment

**Describe the solution you'd like**

At minimum, there should be a way to use Powershell instead of bash with AWS credentials already set. Users should not need hack around tasks to do the things they need to do in a secure way.

**Describe alternatives you've considered**

Add a separate task. This could still be done.

Start by locating the AWSShellScript task entry point and tracing how it currently invokes bash and exposes AWS credentials. Determine how shell selection should work for Powershell without requiring the AWS Tools for Powershell task. Done means the task can securely provide credentials while running the requested non-bash shell, with coverage for the supported behavior.

Written by the indexing model from the issue text.