Add script for usage in IAP public documentation on how to authentica…

[riathakkar]

…tion with IAP using service account JWT

Add scripts to show users how to authenticate with IAP with service accounts

• with IAM credentials API and ADC
• with local key file

Description

Fixes #<ISSUE-NUMBER>

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
• [x] Please merge this PR for me once it is approved

[riathakkar]

Needed for IAP service account JWT public docs

[m-strzelczyk]

@riathakkar you have approval to merge, but I noticed that you didn't include any region tags, that are needed if you want to include these code samples in the public documentation. Is that intentional?

[riathakkar]

@m-strzelczyk what is a region tag?

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 3 region tags.

• iap/generate_self_signed_jwt.py:22, tag iap_generate_self_signed_jwt
• iap/generate_self_signed_jwt.py:48, tag iap_sign_jwt_IAM
• iap/generate_self_signed_jwt.py:69, tag iap_sign_jwt_with_key_file

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