feat: support Gemini API and add unit tests

The implementation includes the following files and classes:
- `GeminiChatModel` in `packages/agentscope/src/model/gemini-model.ts`
- Unit tests in `packages/agentscope/tests/model/gemini-model.test.ts`

The expected functionalities to support:
- Streaming and non-streaming responses
- Tool calling API
- Tool choice

The unit tests should use mock responses to cover the following cases:
- `call` function with:
- Streaming response with tool calling
- Non-streaming response with tool calling
- `_formatToolChoice` method
- `_formatToolSchemas` method

Example implementation refers to [dashscope](https://github.com/agentscope-ai/agentscope-typescript/blob/main/packages/agentscope/src/model/dashscope-model.ts) and its [unit tests](https://github.com/agentscope-ai/agentscope-typescript/blob/main/packages/agentscope/src/model/dashscope-model.test.ts)