fix: use controller's base region for CARM STS AssumeRole calls

[michaelhtm]

Description of changes:
When a resource targets a different region than the controller's home
region, the STS AssumeRole call was incorrectly using the target
region's STS endpoint. This fails when the target is an opt-in region
whose STS endpoint cannot process AssumeRole for the base region's
credentials. With this change, AssumeRole calls are made using the
base region's STS endpoint, and the returned config is then set to the
desired target region for resource management

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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This pull-request has been approved by: michaelhtm

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@michaelhtm: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
verify-attribution	55d3f10	link	false	/test verify-attribution
s3-controller-test	55d3f10	link	true	/test s3-controller-test

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[knottnt]

@michaelhtm Just a minor nit on the actual code change, but I think we'll also need to add a test validate this fix.

[knottnt]

nit: Without additional knowledge it isn't totally clear what the baseRegion and region parameters actually mean. Could you add a description for the function that defines these parameters? Also renaming the parameters to something like defaultControllerRegion and targetRegion/resourceRegion could help.

[knottnt]

An explanation of why both values are needed would also be helpful.