[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/mle.py

@@ -87,6 +87,7 @@ class TlvType(IntEnum):
     ACTIVE_OPERATIONAL_DATASET = 24
     PENDING_OPERATIONAL_DATASET = 25
     THREAD_DISCOVERY = 26
+    CSL_SYNCHRONIZED_TIMEOUT = 85
     TIME_REQUEST = 252
     TIME_PARAMETER = 253
 
@@ -1047,6 +1048,19 @@ class ThreadDiscoveryFactory:
         return ThreadDiscovery(tlvs)
 
 
+class CslSynchronizedTimeout:
+    # TODO: Not implemented yet
+
+    def __init__(self):
+        print("CslSynchronizedTimeout is not implemented yet.")
+
+
+class CslSynchronizedTimeoutFactory:
+
+    def parse(self, data, message_info):
+        return CslSynchronizedTimeout()
+
+
 class TimeRequest:
     # TODO: Not implemented yet