Async support for WebIdentityTokenCredentialsProvider

### Describe the feature

Add support for WebIdentityTokenCredentialsProvider to use an async client

### Use Case

Same use case than in this previous issue : https://github.com/aws/aws-sdk-java-v2/issues/1881

If I understand correctly, the new STSWebIdentityTokenCredentialsProvider didn't add the ability to use an async client. So we still need to add a sync http client on the classpath to use the WebIdentityTokenCredentialsProvider.

### 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 Java SDK version used

2.21.16

### JDK version used

openjdk version "17.0.1" 2021-10-19

### Operating System and version

macOS Monterey

Empieza leyendo WebIdentityTokenCredentialsProvider y STSWebIdentityTokenCredentialsProvider; después revisa el issue 1881 enlazado para el caso de uso del async-client. El issue no indica archivos ni tests; se considera terminado cuando el provider puede usar un async-client sin requerir un cliente HTTP síncrono en el classpath.

Escrito por el modelo de indexación a partir del texto del issue.