[platform] implement otPlatLogOutput platform API (#12762)

This commit provides the platform-level implementation for the
instance-aware logging API `otPlatLogOutput()`. This API is used when
`OPENTHREAD_CONFIG_LOG_INSTANCE_AWARE_API_ENABLE` is enabled, allowing
the platform to receive the `otInstance` pointer with each log line.

The new API is implemented across:
- The simulation platform logging.
- The POSIX platform using `syslog()`.
- The NCP base to route logs to the NCP host.
- The CLI logging module.
- Unit tests and mock platforms.

The `OPENTHREAD_CONFIG_LOG_INSTANCE_AWARE_API_ENABLE` configuration is
also enabled for Toranj simulations to support multi-instance log
testing.

tests/unit/test_srp_server.cpp

@@ -134,11 +134,11 @@ void otPlatFree(void *aPtr)
 #endif
 
 #if OPENTHREAD_CONFIG_LOG_OUTPUT == OPENTHREAD_CONFIG_LOG_OUTPUT_PLATFORM_DEFINED
-void otPlatLog(otLogLevel aLogLevel, otLogRegion aLogRegion, const char *aFormat, ...)
+#if OPENTHREAD_CONFIG_LOG_INSTANCE_AWARE_API_ENABLE
+void otPlatLogOutput(otInstance *, otLogLevel, const char *aLogLine) { printf("   %s\n", aLogLine); }
+#else
+void otPlatLog(otLogLevel, otLogRegion, const char *aFormat, ...)
 {
-    OT_UNUSED_VARIABLE(aLogLevel);
-    OT_UNUSED_VARIABLE(aLogRegion);
-
     va_list args;
 
     printf("   ");
@@ -148,6 +148,7 @@ void otPlatLog(otLogLevel aLogLevel, otLogRegion aLogRegion, const char *aFormat
     printf("\n");
 }
 #endif
+#endif
 
 } // extern "C"