feat(vertexai): Add stream text sample

[ymotongpoo]

Description

Go porting of the following Python sample:
https://github.com/GoogleCloudPlatform/python-docs-samples/blob/main/generative_ai/inference/stream_text_basic.py

Region tag is generativeaionvertexai_stream_text_basic

Checklist

• I have followed Contributing Guidelines from CONTRIBUTING.MD
• Tests pass: go test -v ./.. (see Testing)
• Code formatted: gofmt (see Formatting)
• Vetting pass: go vet (see Formatting)
• 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 1 region tag.

• vertexai/stream-text-basic/streamtextbasic.go:17, tag generativeaionvertexai_stream_text_basic

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[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
https://conventionalcommits.org/

[grayside]

Thanks for the sample, I've asked for a small change to unblock approval. The feedback is identical to what I left on #4148

[grayside]

LGTM, thank you!