docs(samples): add batch_get_effective_iam_policies sample code

[haozhang-google]

Towards b/238046989

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Fixes #479 🦕

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/quickstart_batchgeteffectiveiampolicy.py:22, tag asset_quickstart_batch_get_effective_iam_policies

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