[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.

src/cli/cli.cpp

@@ -132,6 +132,9 @@ const struct Command Interpreter::sCommands[] = {
     {"contextreusedelay", &Interpreter::ProcessContextIdReuseDelay},
 #endif
     {"counters", &Interpreter::ProcessCounters},
+#if OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
+    {"csl", &Interpreter::ProcessCsl},
+#endif
     {"dataset", &Interpreter::ProcessDataset},
 #if OPENTHREAD_FTD
     {"delaytimermin", &Interpreter::ProcessDelayTimerMin},
@@ -1283,6 +1286,44 @@ exit:
     AppendResult(error);
 }
 
+#if OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
+void Interpreter::ProcessCsl(uint8_t aArgsLength, char *argv[])
+{
+    otError error = OT_ERROR_INVALID_ARGS;
+
+    if (aArgsLength == 0)
+    {
+        OutputFormat("Channel: %u\r\n", otLinkCslGetChannel(mInstance));
+        OutputFormat("Period: %u(in units of 10 symbols), %ums\r\n", otLinkCslGetPeriod(mInstance),
+                     otLinkCslGetPeriod(mInstance) * kUsPerTenSymbols / 1000);
+        OutputFormat("Timeout: %us\r\n", otLinkCslGetTimeout(mInstance));
+        error = OT_ERROR_NONE;
+    }
+    else if (aArgsLength == 2)
+    {
+        long value;
+
+        SuccessOrExit(error = ParseLong(argv[1], value));
+
+        if (strcmp(argv[0], "channel") == 0)
+        {
+            SuccessOrExit(error = otLinkCslSetChannel(mInstance, static_cast<uint8_t>(value)));
+        }
+        else if (strcmp(argv[0], "period") == 0)
+        {
+            SuccessOrExit(error = otLinkCslSetPeriod(mInstance, static_cast<uint16_t>(value)));
+        }
+        else if (strcmp(argv[0], "timeout") == 0)
+        {
+            SuccessOrExit(error = otLinkCslSetTimeout(mInstance, static_cast<uint32_t>(value)));
+        }
+    }
+
+exit:
+    AppendResult(error);
+}
+#endif // OPENTHREAD_CONFIG_MAC_CSL_RECEIVER_ENABLE
+
 #if OPENTHREAD_FTD
 void Interpreter::ProcessDelayTimerMin(uint8_t aArgsLength, char *aArgs[])
 {