Add Media CDN Dual Token - Sign Token code snippet

[msampathkumar]

Description

Fixes # http://b/244355824

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Checklist

• [x] I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• [x] README is updated to include all relevant information
• [x] Tests pass: nox -s py-3.9 (see Test Environment Setup)
• [x] 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)
• [x] Please merge this PR for me once it is approved.
• [ ] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 5 region tags.

• media_cdn/snippets.py:26, tag mediacdn_sign_token
• media_cdn/snippets.py:35, tag mediacdn_sign_cookie
• media_cdn/snippets.py:36, tag mediacdn_sign_url
• media_cdn/snippets.py:93, tag mediacdn_sign_url_prefix
• media_cdn/snippets.py:189, tag mediacdn_sign_token

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[dandhlee]

@msampathkumar Friendly ping on this PR!

[msampathkumar]

Hi @dandhlee, Thank you for the ping

[msampathkumar]

Working on a fresh PR for the same branch!