feat: Add CoAP protocol support to serial modem

- Implement sm_at_coap.c with CoAP protocol handling
  - Support for multiple CoAP contexts (up to 3 simultaneous connections)
  - CoAP socket management and message handling
  - TLS/DTLS credential support
  - CoAP payload buffering (512 bytes)

- Update CMakeLists.txt to include CoAP source file
  - Add conditional compilation with CONFIG_COAP flag
  - Integrate sm_at_coap.c into the build system

This enables CoAP protocol communication capabilities in the serial modem application.

Author: zavdimka

app/CMakeLists.txt

@@ -28,6 +28,7 @@ target_sources_ifdef(CONFIG_SM_CMUX app PRIVATE src/sm_cmux.c)
 target_sources_ifdef(CONFIG_SM_GNSS app PRIVATE src/sm_at_gnss.c)
 target_sources_ifdef(CONFIG_SM_NRF_CLOUD app PRIVATE src/sm_at_nrfcloud.c)
 target_sources_ifdef(CONFIG_SM_MQTTC app PRIVATE src/sm_at_mqtt.c)
+target_sources_ifdef(CONFIG_COAP app PRIVATE src/sm_at_coap.c)
 target_sources_ifdef(CONFIG_SM_MODEM_TRACE_BACKEND_CMUX app PRIVATE src/sm_trace_backend_cmux.c)
 
 add_subdirectory_ifdef(CONFIG_SM_CARRIER src/lwm2m_carrier)