[otns] add default (weak) OTNS platform API implementation in core (#12308)

A default OTNS platform API function otPlatOtnsStatus() is now
provided that writes the status push to the log always (regardless of
configured log level). This is useful as a default handling for apps
built with -DOT_OTNS=ON, avoiding linker errors while not mandating
each platform to implement the API by itself.

Specifically it enables the Posix CLI app ot-cli to be built with OTNS
support, which is required to run Posix nodes in OTNS and receive the
status-push events via stdout logging.

The logging is emitted from the module named "Otns" so that a
simulator or other tool can easily detect the OTNS format status push
events in the log output.

tests/gtest/fake_platform.cpp

@@ -666,9 +666,5 @@ void otPlatDnssdStartRecordQuerier(otInstance *, const otPlatDnssdRecordQuerier
 
 void otPlatDnssdStopRecordQuerier(otInstance *, const otPlatDnssdRecordQuerier *) {}
 
-#if OPENTHREAD_CONFIG_OTNS_ENABLE
-void otPlatOtnsStatus(const char *aStatus) { OT_UNUSED_VARIABLE(aStatus); }
-#endif
-
 void otPlatAssertFail(const char *, int) {}
 } // extern "C"