feat(genaisdk): generate text content from text prompt

[Valeriy-Burlaka]

Description

Creates a new directory for hosting samples for the new GenAI SDK and adds 1st code sample for text generation - googlegenaisdk_textgen_with_txt

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.

• genai/text_generation/textgen_with_txt.go:18, tag googlegenaisdk_textgen_with_txt

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