feat: gemma2 samples with accelerated TPU and GPU

[BigBlackWolf]

Description

Base sample on python GoogleCloudPlatform/python-docs-samples#12598

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 2 region tags.

• gemma2/gemma2_predict_gpu.go:17, tag generativeaionvertexai_gemma2_predict_gpu
• gemma2/gemma2_predict_tpu.go:17, tag generativeaionvertexai_gemma2_predict_tpu

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[rsamborski]

Before moving forward with this PR, we need to figure out a way to not include the interface in the sample - it's used only for testing purposes and shouldn't overcomplicate the sample code.

[BigBlackWolf]

I moved interface under the sample, out of the region tag. Additionally, described the way of initializing and closing real one in comment to the function.

[rsamborski]

@telpirion WDYT? Can we merge or would recommend additional changes?

[telpirion]

I've rewritten this sample to more closely match samples style. I'll approve and merge shortly.

Approving and merging sample