feat: Elastic Text-Embedding Model demo (preview0815).

[kewent]

Description

Fixes: http://b/349444341

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Checklist

• [x] I have followed Contributing Guidelines from CONTRIBUTING.MD
• [x] Tests pass: go test -v ./.. (see Testing)
• [x] 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
• [x] Please merge this PR for me once it is approved

[snippet-bot[bot]]

Here is the summary of changes.

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- [aiplatform/snippets/embeddings_preview.go:17](https://github.com/kewent/golang-samples/blob/8e1a721b713211fa70108e56d892be7d51e49e6c/aiplatform/snippets/embeddings_preview.go#L17), tag `generativeaionvertexai_text_predictions`

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- [aiplatform/snippets/embeddings_preview.go:17](https://github.com/GoogleCloudPlatform/golang-samples/blob/66fabfc8f7e333f244e80e811c0110f4647d1316/aiplatform/snippets/embeddings_preview.go#L17), tag `generativeaionvertexai_sdk_embedding`

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