KerasHub integration for local inference

**Overview**
Implement KerasHub integration into Google ADK, providing local model inference capabilities alongside the existing LiteLLM remote model support. This enables users to run AI agents completely offline without requiring external API calls.

**Key Features**
- Local Model Support
- 50+ models supported across multiple families (GPT-2, OPT, BLOOM, LLaMA, Gemma, BERT, T5, etc.)
- No external API calls during inference - runs entirely on local hardware
- Privacy-focused - all computation happens locally
- Cost-effective - no per-token charges