[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/config.py

@@ -257,6 +257,8 @@ def create_default_mle_tlvs_factories():
         mle.TlvType.TIME_REQUEST: mle.TimeRequestFactory(),
         mle.TlvType.TIME_PARAMETER: mle.TimeParameterFactory(),
         mle.TlvType.THREAD_DISCOVERY: create_default_mle_tlv_thread_discovery_factory(),
+        mle.TlvType.LINK_METRICS_QUERY: mle.LinkMetricsQueryFactory(),
+        mle.TlvType.LINK_METRICS_REPORT: mle.LinkMetricsReportFactory(),
     }