[low-power] implement forward tracking series (#5608)

This commit implements the Forward Tracking Series feature in Link Metrics.

- Two new apis are added:
  - otLinkMetricsSendMgmtRequestForwardTrackingSeries, which is used
    to send an MLE Link Metrics Management Request
  - otLinkMetricsSendLinkProbe, which is used to send an MLE Link Probe

- A new class LinkMetricsSeriesInfo is used to maintain the data of
  one Series configured by a neighbor. This class inherits
  LinkedListEntry and each Neighbor has a list of
  LinkMetricsSeriesInfo. All LinkedListEntrys are allocated and freed
  by a Pool in LinkMetrics.

- To specify SeriesFlags in cli command for Forward Tracking Series, a
  similar approach with LinkMetricsFlags is used. Another character
  flags is used to represent SeriesFlags.

- Whenever the node receives a frame (including ACK) from a neighbor,
  it would call Neighbor::AggregateLinkMetrics which goes through each
  LinkMetricsSeriesInfo entry in the neighbor's list and aggregate the
  data if the frame type matches the entry.

- Two test scripts are added to test this function:
  - v1_2_LowPower_7_2_01_ForwardTrackingSeries.py
  - v1_2_LowPower_test_forward_tracking_series.py

tests/scripts/thread-cert/node.py

@@ -1920,6 +1920,22 @@ class NodeImpl:
         self.send_command(cmd)
         self._expect('Done')
 
+    def link_metrics_query_forward_tracking_series(self, dst_addr: str, series_id: int):
+        cmd = 'linkmetrics query %s forward %d' % (dst_addr, series_id)
+        self.send_command(cmd)
+        self._expect('Done')
+
+    def link_metrics_mgmt_req_forward_tracking_series(self, dst_addr: str, series_id: int, series_flags: str,
+                                                      metrics_flags: str):
+        cmd = "linkmetrics mgmt %s forward %d %s %s" % (dst_addr, series_id, series_flags, metrics_flags)
+        self.send_command(cmd)
+        self._expect('Done')
+
+    def link_metrics_send_link_probe(self, dst_addr: str, series_id: int, length: int):
+        cmd = "linkmetrics probe %s %d %d" % (dst_addr, series_id, length)
+        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)