feat: service account manipulations samples

[BigBlackWolf]

Description

Fixes #

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• README is updated to include all relevant information
• Tests pass: nox -s py-3.9 (see Test Environment Setup)
• Lint pass: nox -s lint (see Test Environment Setup)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• Please merge this PR for me once it is approved

[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• iam/cloud-client/snippets/service_account_get_policy.py:17, tag iam_service_account_get_policy
• iam/cloud-client/snippets/service_account_rename.py:15, tag iam_rename_service_account
• iam/cloud-client/snippets/service_account_set_policy.py:17, tag iam_service_account_set_policy

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[m-strzelczyk]

Great! :) I really like your comments explaining some nuances or pointing out what to look out for.