mobile: track network state (wlan/wwan/unreachable) changes when using c++ engine builder.

*Title*: *[One line description](mobile: track network state (wlan/wwan/unreachable) changes when using c++ engine builder.)*

*Description*:
As it is now, only Swift/obj-c and Kotlin/Java engines builder set up network monitors which inform Envoy about network state changes such as wlan->wwan and wwan->wlan transitions which allows the library to refresh its DNS entries and/or recreate connections as needed.

iOS logic for setting up network monitor:
https://github.com/envoyproxy/envoy/blob/cea17494ef49969b888368a5fe1709e347acea2e/mobile/library/objective-c/EnvoyEngineImpl.mm#L440-L444

Android logic for setting up network monitor:
https://github.com/envoyproxy/envoy/blob/cea17494ef49969b888368a5fe1709e347acea2e/mobile/library/java/io/envoyproxy/envoymobile/engine/AndroidEngineImpl.java#L26