feat: export all policies for an IAM principal

[gt12889]

Closes #500

Summary

• Add the iam_policies command for a collected IAM user or role name/ARN.
• Report inline policies, directly attached managed policies, and group-inherited policies.
• Include policy source attribution and the current managed-policy document when collected.
• Add a network-free unit test covering every policy source and unknown principals.

Usage

python cloudmapper.py iam_policies --accounts <account> --principal <name-or-arn>

Verification

• python -m pytest -q tests/unit/test_iam_policies.py — 3 passed.
• python -m compileall -q commands/iam_policies.py tests/unit/test_iam_policies.py — passed.

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