samples(discoveryengine): Add Grounded Generation API Samples

[holtskinner]

NOTE: This feature is allowlist only, but is visible in the public documentation and needs samples. The Public SDK does not currently include these methods.

https://cloud.google.com/generative-ai-app-builder/docs/grounded-gen

[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• discoveryengine/standalone_apis_sample.py:132, tag genappbuilder_grounded_generation_inline_vais
• discoveryengine/standalone_apis_sample.py:205, tag genappbuilder_grounded_generation_google_search
• discoveryengine/standalone_apis_sample.py:264, tag genappbuilder_grounded_generation_streaming

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[iennae]

NOTE: This feature is allowlist only, but is visible in the public documentation and needs samples. The Public SDK does not currently include these methods.

https://cloud.google.com/generative-ai-app-builder/docs/grounded-gen

Can we allowlist the samples testing project?

[holtskinner]

NOTE: This feature is allowlist only, but is visible in the public documentation and needs samples. The Public SDK does not currently include these methods.
https://cloud.google.com/generative-ai-app-builder/docs/grounded-gen

Can we allowlist the samples testing project?

It's not just the allowlist, but the client libraries don't include the methods yet. So this can't be merged until then.

[davidcavazos]

Samples LGTM. Let's wait until testing gets unblocked before merging.