Support for authentication with IAM Identity Center Application

### Describe the feature

`aws configure sso` / `aws sso login` provide functionality to [obtain a session using IAM Identity Center authentication](https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-sso.html).

However, these commands do not support authentication using IAM Identity Center applications such as “External AWS accounts.”

### Use Case

Like `aws sso login`, it would be convenient if authentication could be performed “easily” without [saml2aws](https://github.com/Versent/saml2aws) or [`aws sts assume-role-with-saml` using SAMLResponse after browser authentication](https://htnosm.hatenablog.com/entry/2023/06/25/090000#AssumeRoleWithSAML-%E3%82%92%E5%88%A9%E7%94%A8%E3%81%99%E3%82%8B).

### 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

### CLI version used

aws-cli/2.27.63 Python/3.13.5 Darwin/24.5.0 source/arm64

### Environment details (OS name and version, etc.)

macOS 15.5 (24F74)