Add support for "Streamble HTTP" transport

[mattt]

See modelcontextprotocol/specification#206

[stallent]

So this is somewhat of an "in the meantime" thing... SSE isnt going away in the updated version of the spec, and we need a server implementation that supports multiple clients connected to it, so working on a minimal hummingbird based server using parts of this framework. Might be useful learnings for the actual implementation when you are ready to tackle it. Already have an SSE client transport working, which was very easy with your transport protocol. Won't be as easy for the server, lol.