Support MCP tool using for multi turn

[AlecHenx]

We found that the MCP server marketplace provides a wider variety of tools for invocation. Additionally, compared to traditional function calling, MCP offers a more user-friendly way to use tools, as users do not need to worry about the processing and parsing data from the interaction between tools and the LLM.

1. We hope to directly access the MCP server during multi-turn interactions to obtain the results of tool using.
2. Support for custom MCP tools and access to the MCP server marketplace.

[AlecHenx]

cc @feifeibear

[feifeibear]

We can start with the search tools. like https://github.com/RamXX/mcp-tavily