feat: Add support for vertex gemini model optimizer

[calvingiles]

This enables the use of the model-optimizer-* family of models in vertex, as per the documentation.

To use this, ensure your location is set to global and pass a model optimizer model to an agent:

root_agent = Agent(
    model="model-optimizer-exp-04-09",
    name="fast_and_slow_agent",
    instruction="Answer any question the user gives you - easy or hard.",
    generate_content_config=types.GenerateContentConfig(
        temperature=0.01,
        model_selection_config=ModelSelectionConfig(
            feature_selection_preference=FeatureSelectionPreference.BALANCED
            # Options: PRIORITIZE_QUALITY, BALANCED, PRIORITIZE_COST
        )
    ),
)