Env vars to configure IMDS retry and timeouts

### Describe the feature

Other official AWS SDK libraries support specifying `AWS_METADATA_SERVICE_NUM_ATTEMPTS` and `AWS_METADATA_SERVICE_TIMEOUT` environment variables to automatically retry IMDS requests.

This feature is currently missing in the `aws-sdk-rust` making it more difficult for applications to handle rare credential failures originating from IMDS requests.

### Use Case

When applications are deployed in Kubernetes cluster with [KIAM](https://github.com/uswitch/kiam) project intercepting and redirecting IMDS requests, it can be flaky.

Applications such as [Vector](https://github.com/vectordotdev/vector) will out right abort and throw-away the current operation when it encounters an IMDS credential error. It would be super beneficial to have the retry abilities built-in to the SDK itself.

### Proposed Solution

Implement retry logic into the library with `AWS_METADATA_SERVICE_NUM_ATTEMPTS` and `AWS_METADATA_SERVICE_TIMEOUT` environment variables support.

### Other Information

_No response_

### Acknowledgements

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

### A note for the community

### Community Note
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The issue names no files or tests; start by locating the IMDS credential request implementation and its existing environment-variable configuration. Add support for AWS_METADATA_SERVICE_NUM_ATTEMPTS and AWS_METADATA_SERVICE_TIMEOUT, including retry and timeout behavior, and verify the configured behavior with tests.

Written by the indexing model from the issue text.