Provide a way to namespace groups of tools

I'm finding cases where I want to expose only a subset of tools that I loaded into jupyter-server-mcp.

This case comes up when you have multiple agents working in JupyterLab and you'd like them to specialize. 

I'm thinking we should provide a way for there to be namespaces urls, i.e. `/mcp/group1` would expose one set of tools, preconfigured before the server is launched.