🌱 Configure AWS SDK clients directly, rather than setting environment variables

[TylerGillson]

Summary

We should not be using env vars to implicitly configure AWS SDK clients. We should configure them explicitly using the SDK constructor. Env vars can be defaults (provided by the Helm charts) that get overridden with explicit auth per AWSValidator. This is more idiomatic and prevents contention between multiple AWSValidators reconciled concurrently with differing auth configuration. Currently that is not an issue because we don't support configuring MaxConcurrentReconciles, but is a good improvement nevertheless.