[low-power] implement link metrics - single probe (#5481)

This commit is the first part in link metrics. It implements single
probe function.

An overview of major changes:
- Add link metrics apis which allow apps to send a single probe and
  set report callback.
- Add cli commands for calling link metrics api and related README
  doc.
- Add a new module link_metrics to implement the query process and
  handle the report.
- Add related tlv types.
- Add simple script test to run single probe process.

tests/scripts/thread-cert/mle.py

@@ -89,6 +89,8 @@ class TlvType(IntEnum):
     THREAD_DISCOVERY = 26
     CSL_CHANNEL = 80
     CSL_SYNCHRONIZED_TIMEOUT = 85
+    LINK_METRICS_QUERY = 87
+    LINK_METRICS_REPORT = 89
     TIME_REQUEST = 252
     TIME_PARAMETER = 253
 
@@ -1102,6 +1104,32 @@ class TimeParameterFactory:
         return TimeParameter()
 
 
+class LinkMetricsQuery:
+    # TODO: Not implemented yet
+
+    def __init__(self):
+        print("LinkMetricsQuery is not implemented yet.")
+
+
+class LinkMetricsQueryFactory:
+
+    def parse(self, data, message_info):
+        return LinkMetricsQuery()
+
+
+class LinkMetricsReport:
+    # TODO: Not implemented yet
+
+    def __init__(self):
+        print("LinkMetricsReport is not implemented yet.")
+
+
+class LinkMetricsReportFactory:
+
+    def parse(self, data, message_info):
+        return LinkMetricsReport()
+
+
 class MleCommand(object):
 
     def __init__(self, _type, tlvs):