feat(genai sdk): count and compute tokens

[Valeriy-Burlaka]

Description

Adds samples for counting and computing tokens using the new GenAI SDK. Added region tags:

• googlegenaisdk_counttokens_with_txt
• googlegenaisdk_counttokens_with_txt_vid
• googlegenaisdk_counttokens_resp_with_txt
• googlegenaisdk_counttokens_compute_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 8 region tags.

• genai/count_tokens/compute_with_txt.go:19, tag googlegenaisdk_count_tokens_compute_with_txt
• genai/count_tokens/compute_with_txt.go:20, tag googlegenaisdk_counttoken_compute_with_txt
• genai/count_tokens/count_with_txt.go:19, tag googlegenaisdk_count_tokens_with_txt
• genai/count_tokens/count_with_txt.go:20, tag googlegenaisdk_counttoken_with_txt
• genai/count_tokens/count_with_txt_vid.go:19, tag googlegenaisdk_count_tokens_with_txt_img_vid
• genai/count_tokens/count_with_txt_vid.go:20, tag googlegenaisdk_counttoken_with_txt_vid
• genai/count_tokens/generate_and_count.go:19, tag googlegenaisdk_count_tokens_resp_with_txt
• genai/count_tokens/generate_and_count.go:20, tag googlegenaisdk_counttoken_resp_with_txt

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[Valeriy-Burlaka]

Following the same comment in the Python repo.