feat(vertexai/genai): constrained decoding (#9731)

Expose FunctionCallingMode, which lets the user configure
how the model issues function calls.

By setting the new field `Model.ToolConfig`, the user can select whether
the model can call any provided function, only those in a provided list,
or no functions.

Regenerating also brought along a few other new types and values,
some of which required configuration.