feat: Add Gemini PDF Sample & Add Type for GenerationConfig

[holtskinner]

Add PDF Sample for https://cloud.google.com/vertex-ai/generative-ai/docs/multimodal/send-multimodal-prompts

• Change existing Gemini Samples to use GenerationConfig class instead of a generic dict.

[snippet-bot]

Here is the summary of changes.

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• generative_ai/gemini_pdf_example.py:16, tag generativeaionvertexai_gemini_pdf

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