[history-tracker] move HistoryTracker to its own namespace (#11756)

This commit refactors `Utils::HistoryTracker` by moving it into its
own dedicated `ot::HistoryTracker` namespace for better code
organization.

The main implementation class is renamed from `HistoryTracker` to
`Local`, making the new class `ot::HistoryTracker::Local`. This
allows additional components (like server and client) to be defined
within the `HistoryTracker` namespace.

Nested types, such as `Iterator`, are now direct members of the new
`HistoryTracker` namespace.

src/core/common/notifier.cpp

@@ -152,7 +152,7 @@ void Notifier::EmitEvents(void)
     Get<Utils::Otns>().HandleNotifierEvents(events);
 #endif
 #if OPENTHREAD_CONFIG_HISTORY_TRACKER_ENABLE
-    Get<Utils::HistoryTracker>().HandleNotifierEvents(events);
+    Get<HistoryTracker::Local>().HandleNotifierEvents(events);
 #endif
 #if OPENTHREAD_ENABLE_VENDOR_EXTENSION
     Get<Extension::ExtensionBase>().HandleNotifierEvents(events);