feat: Recommending using OIDC

[jplock]

Issue #, if available: N/A

Description of changes: We should recommend that customers use OpenID Connect to authenticate instead of long-term credentials

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[peterwoodworth]

Thanks for the PR @jplock, this is a good warning to throw 🙂

I think this will end up throwing a warning when there are no permanent credentials being used - e.g. if you are role chaining. I think if you hide this warning behind a check for the absence of AWS_SESSION_TOKEN then we will only throw this when we want to.

Additionally, we should have this warning in our unit test coverage. At least one unit test that role-chains should explicitly not throw this warning, and at least one unit test should explicitly throw this warning with user credentials