[nexus] introduce simulation observer interface and hooks (#12894)

This commit introduces the `SimulationObserver` interface and integrates
it into the Nexus core simulation logic. This allows external systems to
observe node state changes, link updates, and packet events in real-time.

Key changes:
- Defined `SimulationObserver` interface to handle node state changes,
  link updates, packet events, and event clearing.
- Added `SetObserver` and `GetObserver` methods to the `Core` class.
- Implemented `Core::HandleNeighborTableChanged` to notify the observer
  of neighbor additions and removals.
- Implemented `Core::HandleStateChanged` to track node role transitions
  and parent changes, updating links accordingly.
- Integrated packet event notification in `Core::ProcessRadio`,
  including basic destination node ID resolution for unicast frames.
- Added `Core::SetNodeEnabled` to allow enabling or disabling Thread and
  MLE on specific nodes at runtime.
- Updated `Core::Reset` to clear events via the observer.
- Increased `OPENTHREAD_CONFIG_MAX_STATECHANGE_HANDLERS` to accommodate
  the new nexus state change handler.
- Added `mLastParentId` to `Node` class to correctly manage link updates
  during parent switches or detachment.

tests/nexus/platform/nexus_node.cpp

@@ -28,6 +28,8 @@
 
 #include "nexus_node.hpp"
 
+#include "nexus_utils.hpp"
+
 namespace ot {
 namespace Nexus {
 
@@ -133,9 +135,6 @@ void Node::SendEchoRequest(const Ip6::Address &aDestination,
         messageInfo.SetSockAddr(*aSrcAddress);
     }
 
-    Log("Sending Echo Request from Node %lu (%s) to %s (payload-size:%u)", ToUlong(GetId()), GetName(),
-        aDestination.ToString().AsCString(), aPayloadSize);
-
     SuccessOrQuit(Get<Ip6::Icmp>().SendEchoRequest(*message, messageInfo, aIdentifier));
 }