Add Support for IAM Roles Anywhere CreateSession

### Describe the feature

Add native support for [CreateSession](https://docs.aws.amazon.com/rolesanywhere/latest/userguide/authentication-create-session.html) to the SDK.

### Use Case

We would like to leverage IAM Roles Anywhere to "bootstrap" AWS credentials into our external services that are written in C#. We are unable to use the [precanned credential_process binaries](https://docs.aws.amazon.com/rolesanywhere/latest/userguide/credential-helper.html).

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### AWS .NET SDK and/or Package version used

n/a

### Targeted .NET Platform

.NET Standard 2.0

### Operating System and version

Windows, Linux

No source file, test, or entry point is named. Start with the linked IAM Roles Anywhere CreateSession documentation, then inspect how aws-sdk-net exposes AWS service operations for .NET Standard 2.0; done means native CreateSession support is available for the stated external-service credential use case.

Written by the indexing model from the issue text.