Add url-based mcp servers support

Currently, RWKV_APP does not support connecting to MCP servers at all. It would be beneficial to add functionality that allows users to configure MCP servers using URLs directly, improving quality and abilities of model with ease of integration.  

This enhancement would expand RWKV_APP's use cases as mobile LLM multitool
 Example of url-based mcp server: https://mcp.deepwiki.com