Support custom LiteLlm() providers in ADK Web UI model selector

Right now, the Web UI only lets you choose fixed Gemini models (e.g., two predefined options). There’s no UI entry or configuration to select custom models via the LiteLlm integration, even though ADK can use
LiteLlm(model="your_provider/your_model", base_url=…, api_key=…)

**Expected Behavior**

In the Web UI Model Selector, include an option like:

- Custom Model (LiteLLM)
- Provider/Namespace (text input)
- Model name (text input)
- Base URL (text input)
- API Key / Token (secure input)

So the UI would create a LiteLlm instance behind the scenes, and use that as the agent’s model. This mirrors what the docs show is already supported in the Python side of ADK.