[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/node.py

@@ -1866,6 +1866,11 @@ class NodeImpl:
 
         return router_table
 
+    def link_metrics_query_single_probe(self, dst_addr: str, linkmetrics_flags: str):
+        cmd = 'linkmetrics query %s single %s' % (dst_addr, linkmetrics_flags)
+        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)