[link-metrics] implement link metrics manager (#9375)

This commit implements a new module LinkMetricsManager, which utilizes
the Link Metrics feature to get the Link Metrics data from neighboring
devices.

The commit also adds a few tests to the module:
- Unit Test: tests/unit/test_link_metrics_manager.cpp, will be run in
  `unit-tests` in `unit.yml`.
- Expect Test: tests/scripts/expect/v1_2-linkmetricsmgr.exp, will be
  run in `expects` in `simulation-1.2.yml`.
- Simulation Test:
  tests/scripts/thread-cert/v1_2_LowPower_test_link_metrics_manager.py,
  will be run in `packet-verification-low-power` in
  `simulation-1.2.yml`.

src/core/common/notifier.cpp

@@ -186,6 +186,9 @@ void Notifier::EmitEvents(void)
     // being published (if needed).
     Get<NetworkData::Publisher>().HandleNotifierEvents(events);
 #endif
+#if OPENTHREAD_CONFIG_LINK_METRICS_MANAGER_ENABLE
+    Get<Utils::LinkMetricsManager>().HandleNotifierEvents(events);
+#endif
 
     for (ExternalCallback &callback : mExternalCallbacks)
     {