Support assume role external ID in STSProfileCredentialsProvider.

[teo-tsirpanis]

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Description of changes:

This PR enhances STSProfileCredentialsProvider to support reading external_id values from config files and if provided, supply them to the AssumeRole operation.

Let me know what kind of testing I should add for this.

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• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
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