AWS secrets: add support for STS session tags

[benashz]

Adds support for configuring session tags for assume role operations.

This PR takes over the work started in #18813

See #18813 (comment) for details.

TODO only if you're a HashiCorp employee

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  of a public function, even if that change is in a CE file, double check that
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  break any tests. Sometimes ENT only tests rely on public functions in CE
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[github-actions]

CI Results:
All Go tests succeeded! ✅

[github-actions]

Build Results:
All builds succeeded! ✅

[vinay-gopalan]

LGTM! Nice test coverage 👍🏼