Feature Request: Support Ollama Models

## Summary

Currently, ADK Go only supports Gemini models through the `model/gemini` package. This issue proposes adding support for [Ollama](https://ollama.ai/), an open-source platform that allows running large language models locally.

## Motivation

Supporting Ollama would provide several benefits:

1. **Local Development**: Enable developers to run agents locally without requiring API keys or internet connectivity
2. **Cost Efficiency**: Use open-source models without API costs
3. **Privacy**: Process sensitive data locally without sending it to external services
4. **Model Variety**: Access a wide range of open-source models (Llama, Mistral, CodeLlama, etc.) through Ollama's unified API
5. **Model-Agnostic Design**: Aligns with ADK's model-agnostic philosophy mentioned in the README

## Current State

- ADK Go currently implements the `model.LLM` interface only for Gemini models
- The `model/gemini` package provides a reference implementation
- Models are created via `gemini.NewModel()` and passed to agents