feat(genai): add text generation samples (4)

[jdomingr]

Add new samples for text generation:

• Text generation with local video.
• Text generation with multiple local images.
• Text generation with multiple images (local image and Gcs image)

All local tests have passed

[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• genai/api/GenAI.Samples/TextGeneration/TextGenWithLocalVideo.cs:17, tag googlegenaisdk_textgen_with_local_video
• genai/api/GenAI.Samples/TextGeneration/TextGenWithMultiImg.cs:15, tag googlegenaisdk_textgen_with_multi_img
• genai/api/GenAI.Samples/TextGeneration/TextGenWithMultiLocalImg.cs:15, tag googlegenaisdk_textgen_with_multi_local_img

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[yyyu-google]

LGTM, thanks