Exposed ExternalId property in AWS options for assumed role session credentials.

[ashishdhingra]

Description

Exposed ExternalId property in AWS options for assumed role session credentials. This is needed for handling The confused deputy problem.

Motivation and Context

Issue #3286

Testing

Modified existing integration test. Dry run executed successfully.

NOTE:

• Didn't test it end-to-end since the ExternalId is handled by underlying .NET SDK.
• Removed DevConfig commit since it is not required for AWSSDK.Extensions.NETCore.Setup. It's also not supported for AWSSDK.Extensions.NETCore.Setup by the build system as of now.

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license