Allow full customization of `AssumeRoleInput` for the AssumeRole credential provider

### Describe the feature

From discussion https://github.com/awslabs/smithy-rs/discussions/1309

Customers should be able to set every field on the `AssumeRole` input without code changes to expose those fields so that if STS adds new fields to the operation, they're immediately accessible after the STS model is updated.

### Use Case

A new field is added to `AssumeRole` and I want to use it when directly constructing an `AssumeRoleProvider`.

### Proposed Solution

Replicate the fields in [`AssumeRoleInput`](https://docs.rs/aws-sdk-sts/0.10.1/aws_sdk_sts/input/struct.AssumeRoleInput.html) in the `AssumeRoleProviderBuilder`.

### Other Information

_No response_

### Acknowledgements

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

### A note for the community

### Community Note
* Please vote on this issue by adding a 👍 [reaction](https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/) to the original issue to help the community and maintainers prioritize this request
* Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do not help prioritize the request
* If you are interested in working on this issue, please leave a comment

Start by comparing the AssumeRoleProviderBuilder with the AWS STS AssumeRoleInput fields linked in the issue. Trace how the builder constructs AssumeRole requests, then ensure every input field can be configured and remains available when the STS model gains fields; validate the resulting provider behavior with the relevant existing tests.

Written by the indexing model from the issue text.