[br-tracker] enhance NetDataBrTracker to support filtering (#11964)

This change enhances `NetDataBrTracker` to support filtering of Border
Routers.

The `NetDataPeerBrTracker` is renamed to `NetDataBrTracker` to reflect
that it can now track all Border Routers, not just peers.

A new `Filter` enum is introduced with `kAllBorderRouters` and
`kExcludeThisDevice` options. This allows callers to specify whether
to include the current device in the list of Border Routers.

The `CountPeerBrs()` and `GetNext()` methods are updated to
`CountBrs()` and `GetNext()` respectively, and now accept a `Filter`
parameter.

This change provides more flexibility to the `NetDataBrTracker` and
makes the code more reusable. The unit tests are also updated to
clean up resources to avoid heap allocation leaks at the end of
tests.

src/core/common/notifier.cpp

@@ -163,7 +163,7 @@ void Notifier::EmitEvents(void)
 #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);
+    Get<BorderRouter::NetDataBrTracker>().HandleNotifierEvents(events);
 #endif
 #endif
 #if OPENTHREAD_CONFIG_SRP_CLIENT_ENABLE