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

src/ModelContextProtocol.AspNetCore/Stateless/StatelessSessionId.cs

@@ -0,0 +1,13 @@
+using ModelContextProtocol.Protocol.Types;
+using System.Text.Json.Serialization;
+
+namespace ModelContextProtocol.AspNetCore.Stateless;
+
+internal sealed class StatelessSessionId
+{
+    [JsonPropertyName("clientInfo")]
+    public Implementation? ClientInfo { get; init; }
+
+    [JsonPropertyName("userIdClaim")]
+    public UserIdClaim? UserIdClaim { get; init; }
+}