Add USB NCM link state control support

This adds the ability to dynamically control the network link state
for NCM devices. The host OS will see the network interface as
connected/disconnected based on the link state.

New API:
- tud_network_link_state(rhport, is_up): Set link up/down state

Example updates:
- Added button control to toggle link state
- Fixed LWIP integration to properly handle link state changes
- Added printf to show correct protocol (NCM vs RNDIS/ECM)

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

examples/device/net_lwip_webserver/src/lwipopts.h

@@ -58,6 +58,7 @@
 #define LWIP_HTTPD_SSI_INCLUDE_TAG      0
 
 #define LWIP_SINGLE_NETIF               1
+#define LWIP_NETIF_LINK_CALLBACK        1
 
 #define PBUF_POOL_SIZE                  4