Add stateless Streamable HTTP support (#392)

- It allows a single MCP session spanning multiple requests
  to be handled by different servers without sharing state
- It does require the servers share data protection keys, but this
  is standard for ASP.NET Core cookies and antiforgery as well
- It immediately throws for unsupported operations like sampling

tests/ModelContextProtocol.AspNetCore.Tests/StreamableHttpClientConformanceTests.cs

@@ -31,7 +31,7 @@ public class StreamableHttpClientConformanceTests(ITestOutputHelper outputHelper
             Services = _app.Services,
         });
 
-        _app.MapPost("/mcp", async (JsonRpcMessage message) =>
+        _app.MapPost("/mcp", (JsonRpcMessage message) =>
         {
             if (message is not JsonRpcRequest request)
             {