fix(credential-provider-node): pass client region to inner credential client region

[kuhe]

Issue

related to #5749
related to #5755

Description

Currently, some credential providers have an "inner" or additional client not instantiated by the user. One example is the instantiation of an STS client for assumeRole, which is available to any SDK client without user input.

This inner client does not inherit configuration from the user instantiated, or "outer" client. That is because the credentialProvider functions (factories) are standalone and may be called outside of the context of an SDK client.

This PR passes in a reference to the outer client config (and its region) in one special case.

• if the default credential provider is used without specification, i.e.

new Client({});
// only if no credentials are provided and STS::AssumeRole is called by the default credential provider

In this case, the outer client's region will be used as the fallback if the partition is not aws, such as govcloud. If the partition is aws, then us-east-1 will be used as the fallback.

See https://docs.aws.amazon.com/IAM/latest/UserGuide/reference-arns.html for partitions.

Testing

We will need to wait for the development of a suite of integration tests for credential providers.

• new integration tests cc @RanVaknin

[github-actions]

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