Make IMcpServer work well for single and multi-client scenarios

[halter73]

@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?

Originally posted by @stephentoub in #45

We've already added server-side SSE transports as requested by #45 in the form of SseResponseStreamTransport and HttpListenerSseServerTransport, but we have not yet figured out the best API shape to support both single client and multi-client scenarios.

We will likely want a separate ASP.NET Core specific package to handle multiple concurrent clients as suggested by #42, but that isn't strictly required to address this issue.

[normj]

At AWS we have a library that allows deploying ASP.NET Core base applications as AWS Lambda functions for.a serverless scenario. If this package was using ASP.NET Core as the HTTP backend instead of HttpListener we would should be able to deploy a .NET MCP server using this library as a serverless application on AWS Lambda.