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

[jdisanti]

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 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

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[jdisanti]

Seems like customers should also be able to configure the underlying STS client when they create the AssumeRoleProvider.