refactor(sdkv3): migrate IAM client to v3

[Hweinstock]

Problem

Iam uses v2.

Solution

• migrate it.
• rename from DefaultIamClient -> IamClient.
• add stronger type assertions on request to avoid weak type assertions later on. (replace as with proper type checks when possible)
• Continue pattern of wrapping sdk types in "safe" types to avoid assertions.

---

• Treat all work as PUBLIC. Private feature/x branches will not be squash-merged at release time.
• Your code changes must meet the guidelines in CONTRIBUTING.md.
• License: I confirm that my contribution is made under the terms of the Apache 2.0 license.

[Hweinstock]

/runIntegrationTests