[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`.

tests/scripts/thread-cert/node.py

@@ -3257,6 +3257,12 @@ class NodeImpl:
         self.send_command(cmd)
         self._expect_done()
 
+    def link_metrics_mgr_set_enabled(self, enable: bool):
+        op_str = "enable" if enable else "disable"
+        cmd = f'linkmetricsmgr {op_str}'
+        self.send_command(cmd)
+        self._expect_done()
+
     def send_address_notification(self, dst: str, target: str, mliid: str):
         cmd = f'fake /a/an {dst} {target} {mliid}'
         self.send_command(cmd)