`Host` abstraction for multiple client → server connections in one process

[jspahrsummers]

Multiplexing has recurred for me a few times now—clients often need to connect to multiple servers simultaneously. Might be worth building something for it into the client SDK.

I think it's not too hard (send all list commands to all servers, then associate the IDs you get back with the server for follow-up gets, reads, calls), it's just annoying boilerplate.

Maybe we build an actual Host abstraction into the SDK, which has multiple Clients?

See also https://github.com/modelcontextprotocol/typescript-sdk/issues/6.