[border-router] decouple NetDataPeerBrTracker from RoutingManager (#11960)

Moves the nested `RoutingManager::NetDataPeerBrTracker` class into its
own standalone class `BorderRouter::NetDataPeerBrTracker` in new
files `br_tracker.cpp` and `br_tracker.hpp`.

`NetDataPeerBrTracker` is now instantiated as a member of `Instance`
and is no longer owned by `RoutingManager`.

There is no logical/behavioral change in the `NetDataPeerBrTracker`
functionality.

Public C APIs `otBorderRoutingGetNextPeerBrEntry()` and
`otBorderRoutingCountPeerBrs()` are updated to get and use the
`NetDataPeerBrTracker` component.

src/core/common/notifier.cpp

@@ -162,6 +162,9 @@ void Notifier::EmitEvents(void)
 #endif
 #if OPENTHREAD_CONFIG_BORDER_ROUTING_ENABLE
     Get<BorderRouter::RoutingManager>().HandleNotifierEvents(events);
+#if OPENTHREAD_CONFIG_BORDER_ROUTING_TRACK_PEER_BR_INFO_ENABLE
+    Get<BorderRouter::NetDataPeerBrTracker>().HandleNotifierEvents(events);
+#endif
 #endif
 #if OPENTHREAD_CONFIG_SRP_CLIENT_ENABLE
     Get<Srp::Client>().HandleNotifierEvents(events);