Implement InMemory Transport Layer for StellaOps Router

- Added InMemoryTransportOptions class for configuration settings including timeouts and latency.
- Developed InMemoryTransportServer class to handle connections, frame processing, and event management.
- Created ServiceCollectionExtensions for easy registration of InMemory transport services.
- Established project structure and dependencies for InMemory transport library.
- Implemented comprehensive unit tests for endpoint discovery, connection management, request/response flow, and streaming capabilities.
- Ensured proper handling of cancellation, heartbeat, and hello frames within the transport layer.

src/__Libraries/StellaOps.Router.Common/Models/HeartbeatPayload.cs

@@ -0,0 +1,34 @@
+using StellaOps.Router.Common.Enums;
+
+namespace StellaOps.Router.Common.Models;
+
+/// <summary>
+/// Payload for the Heartbeat frame sent periodically by microservices.
+/// </summary>
+public sealed record HeartbeatPayload
+{
+    /// <summary>
+    /// Gets the instance ID.
+    /// </summary>
+    public required string InstanceId { get; init; }
+
+    /// <summary>
+    /// Gets the health status.
+    /// </summary>
+    public required InstanceHealthStatus Status { get; init; }
+
+    /// <summary>
+    /// Gets the current in-flight request count.
+    /// </summary>
+    public int InFlightRequestCount { get; init; }
+
+    /// <summary>
+    /// Gets the error rate (0.0 to 1.0).
+    /// </summary>
+    public double ErrorRate { get; init; }
+
+    /// <summary>
+    /// Gets the timestamp when this heartbeat was created.
+    /// </summary>
+    public DateTime TimestampUtc { get; init; } = DateTime.UtcNow;
+}