[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/unit/test_platform.cpp

@@ -592,20 +592,28 @@ void otPlatFlashWrite(otInstance *aInstance, uint8_t aSwapIndex, uint32_t aOffse
 }
 
 #if OPENTHREAD_CONFIG_TIME_SYNC_ENABLE
-uint64_t otPlatTimeGet(void)
-{
-    struct timeval tv;
-
-    gettimeofday(&tv, nullptr);
-
-    return (uint64_t)tv.tv_sec * 1000000 + (uint64_t)tv.tv_usec;
-}
-
 uint16_t otPlatTimeGetXtalAccuracy(void)
 {
     return 0;
 }
-#endif // OPENTHREAD_CONFIG_TIME_SYNC_ENABLE
+#endif
+
+#if OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
+otError otPlatRadioEnableCsl(otInstance *aInstance, uint32_t aCslPeriod, const otExtAddress *aExtAddr)
+{
+    OT_UNUSED_VARIABLE(aInstance);
+    OT_UNUSED_VARIABLE(aCslPeriod);
+    OT_UNUSED_VARIABLE(aExtAddr);
+
+    return OT_ERROR_NONE;
+}
+
+void otPlatRadioUpdateCslSampleTime(otInstance *aInstance, uint32_t aCslSampleTime)
+{
+    OT_UNUSED_VARIABLE(aInstance);
+    OT_UNUSED_VARIABLE(aCslSampleTime);
+}
+#endif // OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
 
 #if OPENTHREAD_CONFIG_OTNS_ENABLE
 void otPlatOtnsStatus(const char *aStatus)