Adds async pathways for assuming a role.

Description

Adds async version of AssumeRoleAWSCredentials.GenerateNewCredentials and subsequent method call chain.

Motivation and Context

There was no async pathway for assuming a role. This is causing issues on platforms that do not support calling HttpClient.Send such as Android and iOS.

This should fix #3626.

Testing

All tests in UnitTests.NetStandard and AWSSDK.ProtocolTests.NetStandard pass.

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