IMDS retries (#3975)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
Addressing https://github.com/awslabs/aws-sdk-rust/issues/1233

## Description
<!--- Describe your changes in detail -->
Add ability to configure the retry classifier on the IMDS client.

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->
Added new unit test to ensure that a user defined retry classifer is
being used.

## Checklist
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rather than leaving it unchecked -->
- [x] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.

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