feat: add nextjs sample

[LukeSchlangen]

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 guidelines from CONTRIBUTING.MD and Samples Style Guide
• Tests pass: npm test (see Testing)
• Lint pass: npm run lint (see Style)
• 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 pull request is from a branch created directly off of GoogleCloudPlatform/nodejs-docs-samples. Not a fork.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• This sample adds a new sample directory, and I created GitHub Actions workflow 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 possible violations 😱

There are 2 possible violations for not having product prefix.

• frontend/run/nextjs-helloworld/app/layout.jsx:16, tag run_nextjs_helloworld_service
• frontend/run/nextjs-helloworld/app/page.jsx:16, tag run_nextjs_helloworld_service

The end of the violation section. All the stuff below is FYI purposes only.

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Here is the summary of changes.

You are about to add 4 region tags.

• frontend/run/nextjs-helloworld/app/layout.jsx:15, tag cloudrun_nextjs_helloworld_service
• frontend/run/nextjs-helloworld/app/layout.jsx:16, tag run_nextjs_helloworld_service
• frontend/run/nextjs-helloworld/app/page.jsx:15, tag cloudrun_nextjs_helloworld_service
• frontend/run/nextjs-helloworld/app/page.jsx:16, tag run_nextjs_helloworld_service

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[LukeSchlangen]

This is likely not the right place for this code. Thanks for sharing this next.js demo with me @glasnt! I'll use that instead.