How do I implement button-triggered provisioning (enable or disable provisioning via a hardware button)?

By default, RainMaker examples enter provisioning mode automatically on power-up. To trigger provisioning via a button:

1. In the app_network_start function, delay the call to network_prov_mgr_start_provisioning until a button press event is detected.
2. To stop provisioning, call network_prov_mgr_stop_provisioning. To re-enter provisioning mode after stopping, the device must be rebooted, and network_prov_mgr_start_provisioning called again on boot.