Add LiteLlmAgent class with documentation

[ThanabordeeN]

Pull Request: Introduce LiteLlmAgent for streamlined model integration

Description

This PR introduces the LiteLlmAgent class to provide a more streamlined approach for integrating with LiteLLM-supported models. The new implementation reduces complexity by wrapping model initialization details within the agent class itself, allowing developers to specify model providers and versions with simple strings rather than constructing separate model objects.

Changes

• Added LiteLlmAgent class that handles LiteLLM model initialization internally
• Simplified agent creation by allowing direct string references to models
• Removed the need to manually instantiate model objects before agent creation
• Updated documentation with usage examples

Before

from google.adk.agents import LlmAgent
from google.adk.models.lite_llm import LiteLlm

agent = LlmAgent(
    model=LiteLlm(model="openai/gpt-4o"),
    name="openai_agent",
    instruction="You are a helpful assistant powered by GPT-4o.",
    # ... other parameters
)

After

from google.adk.agents import LiteLlmAgent

agent = LiteLlmAgent(
    name="general_agent",
    model="openai/gpt-4o",
    description="Agent that answers questions and provides information.",
    instruction="You are a helpful assistant.",
    # ... other parameters
)

[hangfei]

LiteLlm can be added as a model class so this is already supported. Closing this now.