feat: implement MCP Client

[longfin]

This PR adds Model Context Protocol(MCP) client feature, to enable leveraging various MCP servers.

• Due to the nature of web-based projects, I want to exclude stdio transport and only support SSE transport.
• Set the SSE endpoint as an environment variable beginning with the MCP_SSE_ prefix.
• I'm thinking about integrating the basic UI, but it would be better to handle them into other PR due to size.

Here are the details of this work I am thinking about.

• Configuration
• Toolset conversion
• LLM integration (llmcall, chat, summary)
• UI Integration

Since I don't fully understand this project and its tech stack, there may be some inappropriate technology use or approaches. so, if you see anything weird, please feel free to let me know!