[low-power] add csl feature for Thread 1.2 (#4557)

This commit implements the CSL feature in Thread 1.2.

- Add macro definitions for low power to control the compiling of
  source code.

- Add data and methods for running CSL in Mac and SubMac. This mainly
  includes setting CSL parameters, starting/stopping CSL, and the
  timer handling process.

- Add otPlatTimeGetAPI and implementation.

- Add CSL transmission implementation. CSL transmission is a new kind
  of transmission, the related definition and implementaion for the
  whole transmitting process is added.

- Add calling of start/stop CSL in certain cases.

- Implement CSL synchronization maintainence. If a CSL cordinator
  didn't get a frame containing CSL IE for CSLTimeout, the CSL
  receiver is regarded as de-synchronized.

- Add Cli interface for using CSL.

- Implement enhanced Ack with IE. The original code can only generate
  auto ack for Imm-Ack. As CSL requires CSL IE included in enhanced
  ack. This PR implements it.

- Add basic functional test for CSL transmission. More tests
  corresponding to test plan would be added later.

tests/scripts/thread-cert/node.py

@@ -660,6 +660,22 @@ class Node:
         self.send_command('pollperiod %d' % pollperiod)
         self._expect('Done')
 
+    def get_csl_info(self):
+        self.send_command('csl')
+        self._expect('Done')
+
+    def set_csl_channel(self, csl_channel):
+        self.send_command('csl channel %d' % csl_channel)
+        self._expect('Done')
+
+    def set_csl_period(self, csl_period):
+        self.send_command('csl period %d' % csl_period)
+        self._expect('Done')
+
+    def set_csl_timeout(self, csl_timeout):
+        self.send_command('csl timeout %d' % csl_timeout)
+        self._expect('Done')
+
     def set_router_upgrade_threshold(self, threshold):
         cmd = 'routerupgradethreshold %d' % threshold
         self.send_command(cmd)