[otci] add OpenThread Controller Interface library (#5779)

This commit introduces the Proof of Concept implementation of the
OpenThread Controller Interface (OTCI) which can be used to connect
and manage various kinds of OT devices.

This commit also adds the tests to verify that OTCI works for both
simulation and real OT devices.

- Device Connectors
  - CLI Virtual Time Simulation
  - CLI Real Time Simulation
  - CLI SOC Device
  - NCP Virtual Time Simulation (not all APIs work)
  - Real device via SSH
- OTCI tests of simulation devices are executed in Github Actions
- OTCI tests of real devices can be executed like below:

# Install otci python library
cd tests/scripts/thread-cert/otci && python3 setup.py install --user
# Test OT CLI SOC device at /dev/ttyACM0
REAL_DEVICE=1 OT_CLI_SERIAL=/dev/ttyACM0 python3 tests/test_otci.py
# Test OTBR device via SSH
REAL_DEVICE=1 OTBR_SSH=172.16.243.151 python3 tests/test_otci.py

tests/scripts/thread-cert/simulator.py

@@ -147,6 +147,8 @@ class VirtualTime(BaseSimulator):
     RADIO_ONLY = os.getenv('RADIO_DEVICE') is not None
     NCP_SIM = os.getenv('NODE_TYPE', 'sim') == 'ncp-sim'
 
+    _message_factory = None
+
     def __init__(self, use_message_factory=True):
         super(VirtualTime, self).__init__()
         self.sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)