Add support for missing schema types like anyOf

[icnahom]

Description of the feature request:

Add support for anyOf response schema type, it's currently missing in the Dart SDK but it's available in the Python SDK.

What problem are you trying to solve with this feature?

Once schema types supported by Gemini are added to this package, developers will be able to utilize them as needed.

Any other information you'd like to share?

Here are all the schema types: https://cloud.google.com/vertex-ai/generative-ai/docs/multimodal/control-generated-output#fields