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/StreamableHttpServerIntegrationTests.cs

@@ -14,7 +14,7 @@ public class StreamableHttpServerIntegrationTests(SseServerIntegrationTestFixtur
     protected override SseClientTransportOptions ClientTransportOptions => new()
     {
         Endpoint = new Uri("http://localhost/"),
-        Name = "TestServer",
+        Name = "In-memory Streamable HTTP Client",
         UseStreamableHttp = true,
     };