feat(aiplatform): experimental code samples for Codey API (draft)

[gericdong]

Description

Fixes b279595025

Add experimental code samples for Codey API.
This is a draft. Do not merge.

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 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 changes.

You are about to add 5 region tags.

• ai-platform/snippets/predict-code-chat.js:20, tag aiplatform_sdk_code_chat
• ai-platform/snippets/predict-code-completion-comment.js:20, tag aiplatform_sdk_code_completion_comment
• ai-platform/snippets/predict-code-completion-test-function.js:20, tag aiplatform_sdk_code_completion_test_function
• ai-platform/snippets/predict-code-generation-function.js:20, tag aiplatform_sdk_code_generation_function
• ai-platform/snippets/predict-code-generation-unittest.js:20, tag aiplatform_sdk_code_generation_unittest

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[gericdong]

Closed. Break down to individual PRs.