Add support for IAM: update_assume_role_policy operation

Here is the link to the documentation page: https://docs.aws.amazon.com/IAM/latest/APIReference/API_UpdateAssumeRolePolicy.html

Start with the linked AWS IAM UpdateAssumeRolePolicy documentation, then inspect the repository for existing IAM operations and their entry points. Determine how operations are registered and tested; done means this operation is accepted and behaves consistently with the documented request and response expectations.

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