[backbone-router] add build options for Backbone Router features (#5891)

This commit introduces two configurations for Backbone Router to turn
off corresponding features:

- OPENTHREAD_CONFIG_BACKBONE_ROUTER_DUA_NDPROXYING_ENABLE: 1 by
  default, set to 0 to disable Backbone Router DUA ND Proxying

- OPENTHREAD_CONFIG_BACKBONE_ROUTER_MULTICAST_ROUTING_ENABLE: 1 by
  default, set to 0 to disable Backbone Router Multicast Routing

Both configuration requires OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE.

NOTE: At least one of these two configurations must be ON if
OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE=1(otherwise it's meaningless
to turn on OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE).

tests/unit/test_ndproxy_table.cpp

@@ -28,7 +28,7 @@
 
 #include "openthread-core-config.h"
 
-#if OPENTHREAD_FTD && OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE
+#if OPENTHREAD_FTD && OPENTHREAD_CONFIG_BACKBONE_ROUTER_DUA_NDPROXYING_ENABLE
 
 #include "test_platform.h"
 
@@ -120,4 +120,4 @@ int main(void)
 {
     return 0;
 }
-#endif
+#endif // OPENTHREAD_FTD && OPENTHREAD_CONFIG_BACKBONE_ROUTER_DUA_NDPROXYING_ENABLE