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/tusb_config.h

@@ -85,6 +85,7 @@ extern "C" {
 #endif
 
 // Use different configurations to test all net devices (also due to resource limitations)
+#ifndef USE_ECM
 #if TU_CHECK_MCU(OPT_MCU_LPC15XX, OPT_MCU_LPC40XX, OPT_MCU_LPC51UXX, OPT_MCU_LPC54)
   #define USE_ECM 1
 #elif TU_CHECK_MCU(OPT_MCU_SAMD21, OPT_MCU_SAML21, OPT_MCU_SAML22)
@@ -97,6 +98,7 @@ extern "C" {
   #define USE_ECM 0
   #define INCLUDE_IPERF
 #endif
+#endif
 
 //--------------------------------------------------------------------
 // NCM CLASS CONFIGURATION, SEE "ncm.h" FOR PERFORMANCE TUNING