[border-router] initial implementation of prefix/route management (#5856)

This is the initial implementation of Thread Border Router
prefix/route management with:

- Create & publish off-mesh routable (OMR) prefix for Thread network
  - configure the BR as default route.
  - converge to the smallest OMR prefix if multiple prefixes are
    present at the same time.
- Create & advertise on-link prefix for infrastructure network
  - monitor RA messages on infra link and stops myself if there is
    someone else advertising an on-link prefix.

It currently supports single infrastructure link with possibly
multiple BRs. Support for multiple infrastructure links is left for
future.

tests/unit/test_platform.cpp

@@ -700,4 +700,19 @@ otLinkMetrics otPlatRadioGetEnhAckProbingMetrics(otInstance *aInstance, const ot
 }
 #endif
 
+#if OPENTHREAD_CONFIG_BORDER_ROUTING_ENABLE
+otError otPlatInfraIfSendIcmp6Nd(uint32_t            aInfraIfIndex,
+                                 const otIp6Address *aDestAddress,
+                                 const uint8_t *     aBuffer,
+                                 uint16_t            aBufferLength)
+{
+    OT_UNUSED_VARIABLE(aInfraIfIndex);
+    OT_UNUSED_VARIABLE(aDestAddress);
+    OT_UNUSED_VARIABLE(aBuffer);
+    OT_UNUSED_VARIABLE(aBufferLength);
+
+    return OT_ERROR_FAILED;
+}
+#endif
+
 } // extern "C"