[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

script/check-size

@@ -137,6 +137,7 @@ size_nrf52840_version()
             "DUA=1"
             "MLR=1"
             "CSL_RECEIVER=1"
+            "LINK_METRICS=1"
         )
     fi