Add server-side SSE transports

[eiriktsarpalis]

Originally posted by @geffzhang in PederHP/mcpdotnet#24.

Given the existing stdio-based service support, how do I build an SSE-compatible service?

[stephentoub]

@halter73, @PederHP, I understand there's more work to be done around:

• Making IMcpServer work well for 1:1 and many:1 transports
• Moving the ASP.NET-based support into the libraries

Are those tracked by this issue, or can this be considered closed, given that @PederHP merged the initial SSE implementation?