[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.

etc/gn/openthread.gni

@@ -150,6 +150,9 @@ if (openthread_enable_core_config_args) {
     # Enable legacy network support
     openthread_config_legacy_enable = false
 
+    # Enable link metrics
+    openthread_config_link_metrics_enable = false
+
     # Enable link raw service
     openthread_config_link_raw_enable = false